IP 1/35 
 
 PHYSIOLOGICAL AGE AND 
 SCHOOL ENTRANCE 
 
 BY 
 
 ARTHUR K. BEIK 
 Clark University 
 
 A DLSSERTATION SUBMITTED TO THE FACULTY OF 
 CLARK UNIVERSITY, WORCESTER, MASS., IN PARTIAL 
 FULFILLMENT OF THE REQUIREMENTS FOR THE 
 DEGREE OF DOCTOR OF PHILOSOPHY, AND ACCEPTED 
 ON THE RECOMMENDATION OF WILLIAM H. BURNHAM 
 
 Reprinted from the Pedagogical Seminary 
 September, 1913, Vol. XX, pp. 277-321 
 
PHYSIOLOGICAL AGE AND 
 SCHOOL ENTRANCE 
 
 BY 
 
 ARTHUR K. BEIK 
 
 Clark University 
 
 A DISSERTATION SUBMITTED TO THE FACULTY OF 
 CLARK UNIVERSITY, WORCESTER, MASS.. IN PARTIAL 
 FULFILLMENT OF THE REQUIREMENTS FOR THE 
 DEGREE OF DOCTOR OF PHILOSOPHY, AND ACCEPTED 
 ON THE RECOMMENDATION OF WILLIAM H. BURNHAM 
 
 Reprinted from the Pedagogical Seminary 
 September, 1913, Vol. XX, pp. 277-321 
 
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 Tb« University 
 Uf 23 1811 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE^ 
 
 By Arthur K. Beik, Clark University 
 
 Introduction 
 
 Child study has recognized in a general way alternating, 
 or at least differentiated, periods of development, roughly 
 classified according to predominant characteristics, physical 
 and mental, and located with reference to the age of the 
 child in years. Infancy is the term usually applied to the 
 first year or years of life. Following infancy there comes 
 the period of childhood, pretty sharply marked off at the 
 later limit by the characteristics accompanying the attainment 
 of pubescence. 
 
 The period of childhood is again variously divided by the 
 different authors, the location of the division points depend- 
 ing upon the characteristics chosen as division marks. Be- 
 cause of the grouping of some important factors about those 
 years at which school is usually begun, it is becoming more 
 general to designate this as a natural division point, or per- 
 haps better, nodal period, of development. The time preced- 
 ing this nodal period is referred to as " earlier childhood," 
 while the time from this to the appearance of pubescence is 
 referred to as " later childhood." Later childhood, then, be- 
 
 ^ Pres. G. Stanley Hall suggested the field in which to work out 
 this thesis. The suggestions, advice and helpful criticism received 
 from Dr. W. H. Burnham made its completion possible. For the 
 helpful assistance from both these persons the writer wishes to 
 acknowledge his indebtedness. He wishes also to express his thanks 
 to those who assisted in the collection of material, among whom 
 should be mentioned especially Dr. Theodate L. Smith and the library 
 staff. 
 
278 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 gins approximately at the time of entrance to the graded 
 school and continues up to, or until a short time after, the 
 completion of the graded school work, and might well be 
 characterized as " the age of the grades." 
 
 The fact that it does so nearly coincide with the grade 
 school years makes later childhood a most important period 
 from the view points of child study and pedagogy. It is a 
 period deserving of the closest study, especially as regards 
 the child's development and his fitness to succeed in the tasks 
 and cope with the new situations presented in the school 
 environment. 
 
 Following a brief characterization of the developmental 
 features at the close of this later childhood period, it is our 
 intention to take up in this paper a somewhat more detailed 
 discussion of those developmental features that group them- 
 selves about its beginning. It is our purpose to collect and 
 condense available facts and data from the many investiga- 
 tions that have dealt with particular phases of development 
 in these early years, to relate them in so far as is possible, 
 and to suggest something of their significance. 
 
 What we have chosen to call the later limit of childhood 
 is marked by those special phases of development that accom- 
 pany the attainment of pubescence. It would be useless, for 
 our purpose, to attempt a detailed description of these, though 
 it may be worth while to mention a few of the more important 
 points to which we may wish to refer later. 
 
 Very prominent among the physical changes characteristic 
 of the pubescent period is that of growth, both in height and 
 weight. Growth in height shows a remarkable acceleration, 
 extending over several years. The average time for girls is 
 from the tenth or eleventh to the thirteenth or fourteenth 
 year, while for boys it runs from about the eleventh or twelfth 
 to the fifteenth or sixteenth years. Most curves show a simi- 
 lar acceleration in weight following a little later, and occur- 
 ring also a little earlier in girls than in boys. Both height 
 and weight continue to increase for some time after this rapid 
 acceleration, but at a slower and more uniform rate, so that 
 boys and girls may be said to have almost reached adult size 
 at the close of the rapid pubertal weight increase. 
 
 Closely related to these phenomena of general growth, 
 occur rapid growth of certain parts and organs, rapid changes 
 in body proportions, remarkable additions in muscular power, 
 change in quality and increase in quantity of possible mental 
 achievement, and, perhaps most important of all, the attain- 
 ment of pubescence. 
 
 Dr. Crampton, e. g., would seem to give the factor of 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 279 
 
 pubescence a place of prime importance, since he makes it the 
 basis of grading for " physiological age " during this period 
 (26). 
 
 Data with regard to time of eruption of the teeth seem to 
 indicate a close relationship of dentition to the general stage 
 of advancement of development. From tabulations of the 
 number of canines and molars present, Crampton finds a 
 definite correlation of tooth appearance and weight and of 
 tooth appearance and height (26). The more extended in- 
 vestigations of dentition are of interest in this connection 
 also. Boas and Wissler (11), Berten (8), and Rose {'j']^ 
 all found that the eruption of the teeth was earlier, on the 
 average, for girls than for boys. Recalling the fact of earlier 
 pubescence of girls, the suggestion of a relationship between 
 pubescence and dentition, or perhaps better, the relationship 
 of both dentition and pubescence to the general stage of 
 advancement in development, is strong. There is a problem 
 here worthy of further investigation. More knowledge of 
 the relationship of advancement of dentition at this time to 
 other factors of development ought to be very valuable. 
 
 Studies of the skeletal development, as worked out by 
 Rotch (80), Pryor (70) and others, while not showing cor- 
 relations for ossification of any particular part of the skeleton 
 with pubescence, show in a general way a greater rapidity 
 in girls at this stage of life. Great variability in degree of 
 rapidity is also shown, especially by the work of Rotch and 
 Smith (81) and also by that of Pryor indicated above. 
 
 Variability in the time of these developmental phenomena 
 is everywhere evident. Tabulations of growth all show that 
 the acceleration extends over several years. Variability is 
 fully as great in dentition, as indicated by the tabulations for 
 time of tooth eruption. Time of appearance of pubescence 
 is also extremely variable. Crampton found this factor ap- 
 pearing in boys from 12.75 to 16.25 years of age. 
 
 These main facts concerning the developmental phenomena 
 prominent at the pubescent period are sufficient to indicate 
 that the actual limitation of childhood is not to be referred to 
 a mere temporal reckoning of the length of life, but rather 
 to development. They indicate also that this limit does not 
 coincide with a definitely marked " time line," but rather 
 constitutes a broad " time zone " within which the transition 
 from later childhood over to the earlier stages of the ado- 
 lescent period is accomplished. Within this zone, the varia- 
 bility is sufficiently great, as has been pointed out by Dr. 
 Crampton, Rotch and Smith, Pryor and others, to render grad- 
 ing on the basis of chronological basis alone inade^^juate. 
 
280 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 Finally, this stage of development is reached by girls earlier 
 than by boys, as has been indicated by almost every study of 
 development having to do with this period of life. 
 
 While the close of later childhood is thus pretty clearly 
 marked off by prominent developmental features, there is not 
 at its beginning such a clear cut and astounding transition. 
 Those factors which may be used as marks of development 
 are much harder to distinguish, and therefore to correlate. 
 There is available, however, a considerable body of facts that 
 may assist us in the solution of one or two of the most im- 
 portant problems that have to do with this period, and in the 
 statement of some other problems that need solution. 
 
 Among the important problems are these two : ( i ) Is 
 there, at the age of five or six or seven years, evidence of a 
 transition or nodality of development comparable in any man- 
 ner to the transitional stage at puberty? (2) Is the advance- 
 ment in the two sexes equal at this period of life, and if not, 
 how do they differ? We shall take these up in order. 
 
 Evidences of Nodality 
 
 The School Age. — Turning to the first of the problems just 
 indicated, it may be worth while, before taking up the dis- 
 cussion of actual physical characteristics, to point out that 
 this period of the child's life has for centuries been recog- 
 nized in a practical way as transitional, in that it represented 
 the time for the beginning of formal education. The actual 
 chronological age differed somewhat with different peoples, 
 but for the most part formal training, as distinguished from 
 home training was begun at the age of six or seven years. 
 
 Of China, Douglass wrote (31, p. 165) : " School life 
 commonly begins at the age of six, and the youthful learner 
 is at once set to learn by heart easy text-books which give in 
 short sentences the leading principles of Chinese polity." 
 Monroe states that in the old Greek period the Spartan boy 
 was taken after seven years of training under the direct care 
 of the mother and put under charge of assistants to the paeda- 
 monus, being cared for from that time on in public barracks at 
 public expense (56, p. 74). Similarly, Athenian boys began 
 attendance at school at about the age of seven years (56, pp. 
 82-3). Plato, in his ideal Republic would have the boys taught 
 music and gymnastics from the age of seven on (56, p. I3.s)- 
 Medieval and modern education have fallen into line. The 
 legal age for school entrance in most states, and in most 
 countries also, at the present time, falls within the sixth or 
 seventh year. Thus in the educational practice of most nations 
 it has been found experimentally that at about this time of 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 281 
 
 life the child has reached such a stage of development that he 
 can undertake the tasks involved in education of a formal 
 nature. 
 
 If, now, we turn attention to the more tangible evidences of 
 the stage of development, we are led to several considerations. 
 There are the matters of growth, both in height and weight, 
 dentition, growth of various parts of the body, including 
 skull, brain, larynx, eye, etc. Another group of facts also, 
 related both to neurological structure and to psychology may 
 be added. These have to do with the neuro-muscular control 
 of the child. Finally, a few suggestions may be obtained from 
 the field of pathology. 
 
 Height and Weight. — As regards growth in height and 
 weight, it is generally agreed that the closing years of the later 
 childhood period are years of retarded growth. This con- 
 clusion is based upon the results of numerous extended inves- 
 tigations. Conclusions regarding the earlier years of the 
 period are much less definite, partly because of insufficient 
 data, partly because of a difficulty in interpreting the mean- 
 ing of the growth curves for these years. 
 
 Burk, who summarized the available statistics up to the 
 time of his study, 1898, writes as follows (17, p. 257) : 
 
 " If now we turn to Table A of heights, taking, for example, the 
 larger American studies of Bowditch, Peckham and Porter, we see that 
 the rate of growth is somewhat rapid in the beginning, the sixth or 
 seventh year, and decreases with fluctuations until about ten years in 
 girls and twelve years in boys, when the prepubertal acceleration sets 
 in. This general decrease is to be noticed in all the larger studies, 
 though the year of the beginning of prepubertal increase varies a year 
 or so. The same decrease in rate from six years up to the time of the 
 prepubertal increase is to be observed similarly in the case of girls. 
 Curves constructed from absolute annual increases show, as a rule, in 
 this childhood' period one or two pronounced fluctuations, but they do 
 not occur with a regularity in all charts sufficient to be of assurance 
 that their cause is certainly physiological and not merely statistical. 
 Nevertheless, it will be observed that the curves are by no means 
 regular. . . ." 
 
 Although pointing out thus clearly the irregularities in the 
 curve for these early years, Burk goes on to conclude that, in 
 the absence of determinative data, it is perhaps better to re- 
 gard the period from about six years on as one of a general 
 decrease in growth rate, with one or two minor fluctuations. 
 The investigations of Combe, Landsperger and Carstadt, made 
 upon a comparatively small number of children, but more 
 upon an individual plan, are pointed to as justifying this 
 view (17, p. 258). 
 
 Smedley, in the Chicago investigations, noticed something 
 
282 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 of these irregularities in the early school years. Following 
 his reference to the pubescent acceleration of growth, he 
 writes (84, p. 32) : " The charts seem to show that a similar 
 but less well marked period of activity is present from the 
 beginning of school life to the age of nine." 
 
 Englesperger and Ziegler (32, Bd. i) found that of the 
 children in the first year of school, those between the ages five 
 years and nine months and six years were noticeably smaller, 
 on the average, than those from si'x years and one month to 
 seven years of age. The averages for these periods were : 
 
 Boys Girls 
 
 5 yr. 9 mo. — 6 yr.= 109.65 cm. 106.25 cm. 
 
 6 yr. I mo. — 7 yr.= 11 1.66 cm. 110.77 cm. 
 
 They found considerable differences in height at this period 
 between half year, and even fourth year groups. 
 
 A tabulation comparing the weights of these periods showed 
 that a smaller percentage of those under six years had gained 
 than of those over six years of age. This was on the com- 
 parison of weights eight weeks apart at the beginning of 
 school. 
 
 Stratz (86, p. 66), in Germany, on the basis of rather 
 limited statistics, agrees with a former author, Bartel, in 
 making the first four years a period of bodily " fullness." the 
 years from five to seven, inclusive, a period of " stretching," 
 or spurt of growth, and the years eight to ten a second period 
 of bodily fullness, preceding a second spurt from the age of 
 eleven to fifteen. 
 
 So far as available statistical material is concerned, growth 
 in weight presents a condition similar to growth in height. 
 Without giving the data these tabulations may be character- 
 ized briefly, and their indications may be stated. Most tabu- 
 lations begin with the school age and continue from that 
 time on, thus preventing comparisons with the years that 
 precede. Usually, also, the number of data for the earliest 
 school years are few in number. Again, statistics for weight 
 present an added difficulty in interpretation because of the 
 greater variability in this factor. So far as comparisons of 
 absolute figures may be relied upon, there is only a continuous 
 but variable increase in weight from year to year in the two 
 sexes, up to the age of nine or ten, when a retardation occurs. 
 Boys appear to be slightly superior, throughout the entire 
 course. 
 
 There is slight suggestion of a spurt of growth in the be- 
 ginning school years, but there is not sufficient evidence to 
 establish the point. What is needed here is a collection of 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 283 
 
 more data, beginning with the lower years and continuing 
 through the beginning school years. One cannot see in these 
 studies of height and weight a definite proof of a character- 
 istic of growth at this period. It is a fact, however, that 
 from the larger investigations the results strongly suggest a 
 slight acceleration. It is also to be remembered that most of 
 the tabulations have paid attention to yearly age groups only. 
 Much more accurate would be a comparison of age groups 
 for fourth years or perhaps for months. 
 
 The rather indefinite studies of development, as indicated 
 by increase in growth in height and weight, may be supple- 
 mented by some more definite facts with regard to individual 
 organs or parts of the body that give more distinct evidence 
 of a nodality at the school age. 
 
 Development of Teeth and Jazvs. — The period with which 
 we are here dealing is distinctly a transitional one as regards 
 the development of the teeth and jaws. These present a num- 
 ber of phenomena, transitional as well as developmental in 
 their nature, that group themselves about the years in which 
 the change from first to second dentition occurs. Some of 
 these are very evident, others less so, but altogether they make 
 up such a group of developmental processes, so closely asso- 
 ciated with each other, and also to other phases of physical 
 and mental development that they deserve to be noted con- 
 siderably in detail. 
 
 Aside from this, there are also other well grounded reasons 
 for dwelling somewhat at length upon this topic, i. Anatom- 
 ically, the teeth and jaws are the most important structures 
 in the facial part of the skull. Much of the remaining por- 
 tion is of rather secondary nature, its purpose being to fur- 
 nish place for attachment of muscles or to give proper bracing 
 and support for these parts. 2. On the side of physiological 
 functioning the teeth and jaws are extremely important. 
 Upon them devolves a great part of the preparation of food 
 in the process of digestion, not to mention the part they play 
 in articulate speech. 3. Again, from the view point of hygiene, 
 it may be said that columns of good, sound teeth, so located 
 as to bring their chewing surfaces into proper occlusion, are 
 requisites of good health. The absence of these conditions 
 may lead to any of a number of allied disturbances, to which 
 more detailed reference will be made later. 4. Finally, since 
 they occupy so prominent a place anatomically, the form and 
 general outline of the face are to a great extent dependent 
 upon teeth and jaws. It therefore follows that from the view 
 point of esthetics the proper formation and development of 
 teeth and jaws may not be left unconsidered. Thus we may 
 
284 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 say that from any of the four view points, esthetic, hygienic, 
 functional or structural, development of teeth and jaws de- 
 serves the fullest consideration. We shall attempt to con- 
 sider the topic from all four of these view points in the follow- 
 ing paragraphs devoted to it. 
 
 While most concerned with the transitional phenomena, 
 so much depends upon preceding and following developmental 
 processes that a rapid review of the entire period of dentition 
 may be of value in setting forth more clearly the character- 
 istics of the transitional period. In this review, it is assumed 
 that the reader's general knowledge of form, names, and com- 
 position of the teeth is sufficient to render detailed descrip- 
 tions unnecessary. Since similar knowledge with regard to 
 the jaws, on the other hand, seems to be less general, descrip- 
 tions of the principal structures with which we shall need to 
 deal will be included. For the sake of clearness, the facts 
 with regard to development of the teeth will be presented first, 
 then those with regard to development of the jaws. With 
 these two groups of facts before us, we may next proceed to 
 discuss their relationship and its significance, and arrive at 
 some conclusions. With these hints as to method of proced- 
 ure before us, we turn next to the presentation of the facts. 
 
 Although the eruption of the first or temporary teeth from 
 the gums does not take place until some time after birth, the 
 formation of their crowns begins very early in the developing 
 embryonic jaw. Their calcification is already far advanced 
 at the time of birth. The early formation and development 
 of these is somewhat as follows : Above the gums, along the 
 rudimentary jaw of the embryo, there is formed very early 
 a ridge of epithelial cells. At each of the ten points at which 
 a tooth crown is to be formed, a depression or invagination 
 occurs in this ridge, pushing on downward into the jaw in 
 a line, as it were, or tube, and expanding at the end into a 
 bell-shaped structure, — the enamel organ. Gradually the 
 enamel organ grows down over the top and sides of a small 
 mound-like enlargement that rises up from the deeper tissue, 
 — the dental germ. Again a little later the connection of the 
 enamel organ with the epithelial ridge above is lost, enamel 
 organ and dental germ together become isolated from the 
 surrounding parts and enclosed in a " dental sac," and within 
 this sac the two organs proceed to build up the parts of the 
 tooth crown. Out of the soft cells that make up the enamel 
 organ is formed the enamel ; out of those of the tooth germ 
 is formed the dentine or ivory of which the inner part of the 
 tooth is composed, and within which, again, is the open cavity 
 containing the pulp. 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 285 
 
 The transformation of these soft cells into the hard struc- 
 tures of the crown is accomplished by means of the deposition 
 of lime salts within the cells themselves, and is usually spoken 
 of as a process of calcination or calcification. The processes 
 are not the same in both cases, however. While the cells of 
 the enamel organ become changed into solid crystals, those 
 of the dental germ receive the lime deposits only around the 
 outside in such a way that the resulting structures are elongate 
 tubes. The central, uncalcified portions remain as fine fibrils 
 which are connected with the soft pulp of blood-vessels and 
 nerves that fill the inner cavity of the tooth. The dentine 
 tubes thus formed are of considerable length and extend out- 
 ward radially from the pulp cavity to the enamel " roof " of 
 the tooth. 
 
 The direction in which the calcification proceeds differs in 
 the two organs. It will be remembered that the enamel organ 
 folds over and encloses the dental germ. Now, the calcifica- 
 tion begins at the contiguous surfaces of the two organs and 
 proceeds in both directions. For the enamel cells, then, cal- 
 cification proceeds from the inner surface toward the outer; 
 for the dentine cells, it proceeds from the outer surface in- 
 ward toward the pulp cavity. 
 
 Enamel and dentine differ also in another way. The enamel 
 crystals receive no more nourishment after being once formed. 
 The dentine cells, on the other hand, continue to receive some 
 nourishment throughout the life of the tooth by means of 
 the soft fibrils in the central tubular portions. (St,, p. 31.) 
 
 As the development of a crown proceeds beneath the gums 
 there is deposited around it a layer of bony substance, so 
 that by the time it is completed it is almost enclosed in a 
 bony case or crypt. The top of the crypt is open, though 
 the aperture is not large enough for the crown to pass 
 through without absorption of some of the tissue from its 
 edges. This is what occurs at the time of eruption of the 
 crown. 
 
 After the manner thus described there are formed ten tooth 
 crowns in each of the jaws. Their calcification is not entirely 
 completed by the time of birth, but it is completed within 
 the first few months of life. Then they emerge from the 
 gums to form the " temporary set " that functions during the 
 early years of the child's life. Of these, the incisors and 
 canines are small, as compared with the corresponding teeth 
 that follow. The bicuspids, or milk molars, on the other hand, 
 are larger than the permanent bicuspids that follow. In this 
 way ample provision is made for the mastication of such 
 
286 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 foods as the child is able to digest and assimilate during his 
 early years. 
 
 Some weeks after birth the crowns of the temporary set 
 are completed and begin to erupt from the gums. The first 
 become visible at about the sixth to eighth month of life, 
 usually, and at the age of about two and one-half to three 
 years the child is equipped with a full set of twenty. Ap- 
 pearance is usually in groups, beginning with the central in- 
 cisors and proceeding, in a general way, backward from these, 
 with intervals of time elapsing between the several groups. 
 Witzel reports the usual time for the eruption of these, as 
 given by Thomas and Baume, to be as follows (98, p. — ) : 
 
 Central incisors 6-8 mo. 
 
 Lateral incisors 6-12 mo. 
 
 Anterior bicuspids 12-16 mo. 
 
 Canines 15-20 mo. 
 
 Posterior bicuspids 20-30 mo. 
 
 The basis for this statement is not given, no mention being 
 made of actual investigation. The periods given are those 
 within which the eruption from the gums usually occurs. It 
 is possible for the teeth to appear much later, however, and 
 yet be normal. It is also possible for a few of the crowns 
 to be already visible at birth and the teeth be normal, though 
 this is very often the result of diseased conditions. 
 
 The process by which a tooth crown erupts is somewhat 
 complex. The edges of the opening in the bony ciypt must 
 first be reabsorbed, making room for the crown to push 
 through. The gums that form the covering are next absorbed. 
 While this is going on and the crown gradually pushing up- 
 ward, additions are made to the root from cells at the base 
 of the crown, and, once the final position is attained, the 
 root becomes fastened into place by the bony tissue built up 
 around it. The root canal remains as a rather wide opening 
 for some time, however, the dentine being built up gradually 
 until the root is solid, except for the narrow canal through 
 which the vessels and nerves are admitted to the pulp cavity. 
 
 The tissue built up around the teeth and within which their 
 sockets are found is known as the alveolar border of the 
 jaw or alveolar process. It is developed with the teeth and 
 apparently for the specific purpose of holding them in posi- 
 tion. Fuller description of it may be reserved until later. 
 A description of the means by which the tooth roots are 
 firmly fastened within it, however, is in place at this point. 
 This can best be given in the words of Dr. Angle, taken 
 from his paragraph on "Peridental Membrane" (i, p. 122). 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 287 
 
 " The peridental membrane is a strong, fibrous membrane forming a 
 close, cushion-like investment of the roots of the teeth, and is the 
 medium of attachment between the alveolar process and the cementum. 
 It is composed largely of inelastic connective tissue, and is richly 
 supplied with nutrient vessels, nerves, cells, and glands. Its function 
 is three-fold : 
 
 " First, vital, for the formation of the alveolar process on one side 
 and the cementum on the other. 
 
 " Second, sensory, through which the most delicate touch of the 
 tooth is felt. 
 
 " Third, physical, holding the tooth in position in the alveolar socket, 
 and resisting the movements of the teeth in the various directions. It 
 also supports the soft tissues about the teeth." 
 
 Continuing, Dr. Angle enumerates the kinds of cells of 
 which the peridental membrane is composed as, /, fibroblasts, 
 for the formation of fibers of the membrane ; 2, osteoblasts, 
 for the formation of the alveolar process ; 5, cementoblasts, 
 for forming the cementum ; 4, osteoclasts, for disintegrating 
 calcified tissue; and finally, 5, glands, the function of which 
 is as yet imperfectly understood (i, p. 123). 
 
 Malposition of temporary teeth is rare. When it does occur, 
 it is easily corrected, under normal conditions, by means of 
 the pressure from lips and tongue. But while this is true, 
 abnormal conditions may as readily cause the teeth to be 
 moved from their proper positions and thus bring about a 
 malocclusion. 
 
 But the temporary crowns are not formed alone in the 
 embryonic jaw. The beginnings of the permanent crowns are 
 also present at a very early stage. Very early in the develop- 
 ment of the enamel organ of each of the temporary teeth a 
 second line or tube of epithelial cells pushes down by its 
 lingual side. It appears as a branch of the tube which de- 
 velops into the enamel organ of the temporary tooth. In 
 structure, too, it is similar, and its development, later, after 
 the manner described for the temporary tooth, results in 
 the formation of the crown of the permanent tooth. 
 
 Aside from the ten corresponding to the crowns of the 
 temporary set, there are also six other germs in each jaw, 
 as the beginnings of the molars of the second set. The child 
 at birth, then, has in each jaw the almost completed crowns 
 of the ten teinporary teeth and also the germs of the sixteen 
 permanent teeth at various stages of development. The com- 
 pletion of the permanent crowns is of course later than that 
 of the temporary, their time of most rapid growth, according 
 to Pedley (62) being from birth to three or four years of 
 age. Johnson places the critical period for their calcification 
 at from birth to the age of five years, that of the first perma- 
 nent molars having been begun even before the birth of the 
 child (51). 
 
288 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 The time of these processes is given more in detail by 
 Witzel (98, p. 17) : 
 
 " The calcification of the crowns of the incisive teeth begins between 
 the first and second year and the calcification of the canine tooth in 
 the third year. In the fifth year of age the tuberosities for the second 
 molar tooth are developed and in the ninth those for the third. About 
 the time of the ninth year no remarkable processes of calcification take 
 place in the crowns, except in the wisdom tooth, but the roots are not 
 everywhere fully developed. . . ." 
 
 The ages mentioned are of course only approximate, and 
 vary within wide limits. 
 
 From this it follows that from birth to about five years of 
 age is a critical period for all the permanent set, except the 
 wisdom teeth. During this period the dental germs, many 
 of which have begun development at the time of birth, attain 
 their final form, make their most rapid growth and undergo 
 a great part of the process of calcification. In this connection 
 it should be emphasized that when once formed and com- 
 pletely calcified, these crowns are adult crowns, as large as 
 they will ever be, and subject to neither growth nor repair, 
 so far as the organism to which they belong is concerned. 
 Completion of the crowns is accompanied by enlargement of 
 the alveolar border and growth in some parts of the jaw, 
 with considerable redistribution of the inner structures. 
 
 Thus far we have noted the principal features in the forma- 
 tion and calcification of the temporary crowns, marked the 
 important changes involved in their eruption and in their 
 fixation into position by means of developing root structures 
 and alveoli, and have traced the important steps in the process 
 by which the permanent crowns are formed, developed and 
 calcified within the alveolar portions of the jaws. Each one 
 of these is a developmental feature characteristic of the earlier 
 years, and constitutes a step in the preparation for the coming 
 of the permanent teeth. The time immediately preceding sec- 
 ond dentition is therefore a remarkable one for the child, 
 considered from the view point of his dentition. Within his 
 jaws there are i, a full set of twenty functioning temporary 
 teeth, 2, the practically completed crowns of their twenty suc- 
 cessors, many of which are larger, j, the completed crowns 
 of the four first permanent molars and practically completed 
 crowns of the four second molars, and 4, the only partially 
 developed germs of four wisdom teeth or third molars. This 
 is the normal condition of the child's jaws at the age of five 
 or six years. Transition to the second dentition usually begins 
 very soon afterward. 
 
 In discussing the eruption of the permanent crowns we may 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 289 
 
 first give attention to the time of their appearance, and then 
 turn to the description of the process by which it is brought 
 about. 
 
 Time of appearance of the permanent teeth has been made 
 the object of a number of investigations. Some of these are 
 of no vakie for our purpose, since they do not differentiate 
 the sexes. Of the three available that do, one is not extended, 
 but the other two are sufficiently so to make their results 
 valuable. 
 
 First let us turn to the statistics of Boas and Wissler. Their 
 material was a series of plaster casts taken from the palates 
 of 219 boys and 276 girls. They therefore contain data for 
 the upper jaw only. The following Table A is their table 
 numbered " XI C." giving results in percentages (ii, p. 34) : 
 
 TABLE A. 
 
 Percentage of Children Having Permanent Teeth. 
 
 Age 
 
 Boys 
 
 Girls 
 
 In. 
 Inc. 
 
 Out. 
 Inc. 
 
 Can. 
 
 Bic. 
 
 ist 
 Mol. 
 
 2nd 
 Mol. 
 
 In. 
 Inc. 
 
 Out. 
 Inc. 
 
 Can. 
 
 Bic. 
 
 ist 
 Mol. 
 
 2nd 
 Mol. 
 
 6 
 
 30 
 
 4 
 
 
 
 65 
 
 
 38 
 
 9 
 
 6 
 
 15 
 
 83 
 
 2 
 
 
 
 
 
 7 
 
 40 
 
 6 
 
 3 
 
 3 
 
 83 
 
 
 79 
 
 28 
 
 2 
 
 41 
 
 92 
 
 
 8 
 
 81 
 
 26 
 
 
 19 
 
 100 
 
 
 84 
 
 48 
 
 9 
 
 45 
 
 93 
 
 6 
 
 9 
 
 97 
 
 67 
 
 
 30 
 
 
 7 
 
 100 
 
 59 
 
 9 
 
 45 
 
 100 
 
 2 
 
 10 
 
 100 
 
 63 
 
 33 
 
 66 
 
 
 3 
 
 94 
 
 82 
 
 14 
 
 73 
 
 
 8 
 
 1 1 
 
 
 95 
 
 61 
 
 100 
 
 
 9 
 
 96 
 
 96 
 
 73 
 
 83 
 
 
 20 
 
 12 
 
 
 100 
 
 85 
 
 
 
 38 
 
 100 
 
 100 
 
 86 
 
 100 
 
 
 32 
 
 13 
 
 
 
 91 
 
 
 
 68 
 
 
 88 
 
 88 
 
 
 
 68 
 
 14 
 
 
 
 IOC 
 
 
 
 66 
 
 
 90 
 
 9c 
 
 
 
 90 
 
 15 
 
 
 
 
 
 
 75 
 
 
 100 
 
 IOC 
 
 
 
 100 
 
 Averages and variabilities were computed for the time of 
 appearance of the several teeth and found to be as follows 
 
 (12, p. 35): 
 
 Boys 
 
 Inner incisors 7 • 5 ± i • 5 y- 
 
 Outer incisors 9.5^2.1 
 
 Bicuspids 9.8±i.6 
 
 Canines ii.2±i.4 
 
 Second molars 13.2 ±2.0 
 
 Girls 
 
 7.0 ±1.6 yr. 
 
 8.9± 2.1 
 
 9.0 ± 2.8 
 1 1 . 3 ± I . o 
 12. 8± 1.6 
 
290 
 
 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 
 
 fe 
 1— ] 
 
 _ 
 
 
 
 
 
 
 
 as 
 1^ P. 
 
 
 
 
 
 
 
 
 First 
 Molars 
 
 S 
 1—4 
 
 M '^ 
 
 " 
 
 rO eg 
 
 00 N N ro 
 MM MP) 
 
 T^CO 10 M 
 
 Oh 10 ro 
 
 MM MM 
 
 1 
 
 ;:3 (n. 
 
 CO • 
 
 vO 00 
 
 
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 M <N 
 
 ro *t 
 
 10 ro 
 10 ro 
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 1—5 
 
 
 
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PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 291 
 
 • ro 
 
 t-- 
 
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 T)- 10 
 
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 l-l M 
 
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 r~CO 
 
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 r|- 
 
 Th On 
 
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 00 
 
 
 
 
 
 
 
 
 
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 00 
 
 
 
 
 
 
 
 
 
 Tl-vo 
 
 
 
 
 
 
 
 
 
 N (N 
 
 
 
 
 
 
 
 
 
 vo t^ 
 
 -1- CO 
 
 -f- t-~ 
 
 M -i- 
 
 fo CI 
 
 w ro 
 
 l-( M 
 
 ro 
 
 10 NO 
 
 H 01 
 
 ro -r 
 
 •* I- 
 
 U-, On 
 
 r^ ro 
 
 On "0 
 
 LO t^ 
 
 ro 'J-i 
 
 On m 
 
 
 
 
 
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 M 
 
 M M 
 
 M M 
 
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 M 
 
 M -t 
 
 On -I 
 
 eg Lo 
 
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 rocO 
 
 On m 
 
 On 
 
 '-t- M 
 
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 M *-I 
 
 IN 
 
 CS vO 
 
 l-~ ro 
 
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 t- 
 
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 ro^ 
 
 1000 
 
 t^ ro 
 
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 "too 
 
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 ro M 
 
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 M -+ 
 
 NO On 
 
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 M C< 
 
 H Cl 
 
 M Cl 
 
 M M 
 
 M H 
 
 M ^ 
 
 OvO 
 
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 CO ON 
 
 M 00 
 
 Cl 1-1 
 
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 0, w 
 
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 N M 
 
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 CI 
 
 
 
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 roW3 
 
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 OnO 
 
 tKS 
 
 
 
 
 
 
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 t-. 
 
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 N 1> 
 
 
 
 
 
 
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 IN ro 
 
 
 
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 so t- 
 
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 (N 
 
 N fS 
 
 <S N 
 
 
 
 
 
 
 Tt- 
 
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 10 
 
 cq nO 
 
 On m 
 
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 00 rl- 
 
 r^ 10 
 
 On -t 
 
 oq ro 
 
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 l-H CI 
 
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 ly~;NO 
 
 
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 M M 
 
 h-i M 
 
 
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 h-( M 
 
 
 
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 Sf^ 
 
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 ^^ 
 
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 gf^ 
 
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 Ov 
 
 
 
 
 
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 ro 
 
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292 
 
 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 The investigation of Berten was much more extended. He 
 collected data from some 3.345 children in Germany, of ages 
 ranging from five and one-half to thirteen and one-half years. 
 In the accompanying Table B are presented the numbers of 
 teeth present for each half-year group within these ages. 
 This table condenses the data found in Berten's tabulation 
 (8, pp. 278-9). Boldface figures indicate groups where 100 
 per cent, were found present. 
 
 A report of an investigation much more extaided than 
 either of these appeared in recent years. Dr. Rose has tabu- 
 lated the number of teeth found present in the several groups 
 in case of 41,021 children in Germany, Sweden, Denmark, 
 Holland, Belgium, Bohemia and Switzerland. His tabulation 
 differentiates upper and lower teeth, also sex groups, and 
 gives the averages and variabilities for the several tooth 
 groups. 
 
 Table C is his " Table XIV," in which the results of the 
 investigation are summarized {^J, p. 564) : 
 
 TABLE C. 
 
 Eruption Time of Permanent Teeth in 41,021 School Children 
 
 FROM Germany, Sweden, Denmark, Holland, Belgium, 
 
 Bohemia and Switzerland. 
 
 
 Boys^2 1,139 
 
 Girls=i9,882 
 
 
 Average 
 
 eruption 
 
 time 
 
 Eruption 
 
 time 
 
 varies 
 
 between 
 
 ages 
 
 Average 
 
 eruption 
 
 time 
 
 Eruption 
 
 time 
 
 varies 
 
 between 
 
 ages 
 
 
 Yr. Mo. 
 
 Yr. Mo. 
 
 upper jaw 
 Incisor I... 
 Incisor II .. . 
 
 Canine 
 
 Premolar I . 
 Premolar II. 
 
 Molar I 
 
 Molar II.... 
 
 7 8 
 
 8 II 
 12 2 
 
 10 5 
 
 11 4 
 6 7 
 
 12 9 
 
 5 
 6 
 
 7 
 6 
 6 
 5 
 9 
 
 5-1 1.5 Yr. 
 -(?) " 
 
 5-15- 
 5-14. 
 5-15- 
 
 - 9-5 " 
 
 -15- 
 
 7 5 ( -3 Mo.) 
 
 8 6 ( -5 " ) 
 
 II 7 ( -7 " ) 
 
 10 I ( -4 " ) 
 
 11 I ( -3 " ) 
 6 6 ( -I " ) 
 
 12 5 ( -4 " ) 
 
 5 
 6 
 
 7 
 6 
 
 7 
 5 
 9 
 
 5-1 1. Yr. 
 
 -(?) " 
 
 -15- " 
 5-14.5 " 
 
 -15- 
 
 -10. " 
 
 -IS- 
 
 LOWER JAW 
 
 Incisor I . . . 
 Incisor II .. . 
 
 Canine 
 
 Premolar I. 
 Premolar II. 
 
 Molar I 
 
 Molar II.... 
 
 6 10 
 
 7 II 
 II 2 
 
 11 3 
 
 12 
 
 6 5 
 12 3 
 
 5 
 6 
 
 7 
 7 
 7 
 5 
 9 
 
 —ID. Yr. 
 -12. " 
 -15- 
 
 -14-5 " 
 -15- 
 -10. " 
 
 -15- 
 
 67 ( -3 Mo.) 
 7 7 ( -4 " ) 
 
 ID 3 (-11 " ) 
 
 10 8 ( -7 " ) 
 
 10 7 ( -5 " ) 
 6 3 ( -2 " ) 
 
 11 9 ( -6 " ) 
 
 5 
 6 
 
 7 
 7 
 7 
 5 
 8 
 
 -II. Yr. 
 -12. 
 
 -14. " 
 -14.5 " 
 -15- 
 - 9- 
 -15- 
 
 Avg. of the 
 total 14 
 eruptions. 
 
 9 lO.O 
 
 
 9 5.4 ( -4.6 Mo.) 
 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 293 
 
 Careful study of these tabulations will show that they are 
 in agreement as to the essential points regarding the time and 
 the order of eruption of the permanent crowns. Each one 
 confirms the prevalent view that transition to second denti- 
 tion begins with the eruption of the first permanent molars. 
 This is evident in the tabulation of Berten, and in the averages 
 from the very extended investigation of Rose. Berten writes 
 that in only three cases did he find the transition to the perma- 
 nent teeth beginning before the appearance of any molars 
 (8, p. 271). 
 
 Incidentally it should be noted that the time for the appear- 
 ance of the first molars is in the seventh year. Berten's table 
 indicates that practically all appear between the age of six 
 years and seven and one-half, while Rose's averages all fall 
 between the ages six years and three months and six years 
 and seven months. 
 
 Again, the more extended investigations of Berten and 
 Rose have furnished abundant evidence in confirmation of the 
 view that the lower teeth appear, on the average, earlier than 
 the upper, except, perhaps, in case of the canines and bicus- 
 pids. In all the investigations a considerable variation in the 
 time of appearance for the several tooth groups is evident. 
 From this it follows that dentition does not run a parallel 
 course in all children. Variation in time of appearance is 
 least in case of the first molars and grows progressively 
 greater with each succeeding tooth group, up to the bicuspids 
 and canines. 
 
 The order in which the permanent crowns appear is repre- 
 sented in these data. According to them it runs : first molars, 
 inner incisors, outer incisors, first bicuspids, after which the 
 order of the remaining groups is somewhat confused. Berten 
 states (8, p. 274) that most often we find these groups ap- 
 pearing in such a way that the lower canine follows the first 
 bicuspids, and is followed, in turn, by the upper second bi- 
 cuspid ; then the lower second bicuspid appears, and the upper 
 canine brings up the rear. 
 
 Finally, before leaving this topic, the point of sex difiference 
 in time of eruption of the teeth should be pointed out. In 
 each of the three tabulations it is clearly evident, beginning 
 with the first molars, at the very beginning-time of second 
 dentition. Rose's results show the girls to be two months 
 earlier in eruption of lower first molars ; one month ahead in 
 eruption of upper first molars, on the average. The actual 
 difference of time is not so evident in Berten's table because 
 of the form in which it is presented, but comparison of the 
 numbers of the teeth present show that the results would be 
 
294 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 in agreement. Presented in the form of curves showing the 
 percentages of teeth present in the various groups, Berten's 
 data show very clearly that this is true. 
 
 The processes by which the eruption of the permanent 
 teeth is accomplished deserve some further consideration. 
 Evidently the temporary structures must first be disposed of. 
 This is the first step in the eruption of the permanent set. 
 It is accomplished by a sort of reverse process to that by 
 which the temporary roots are builded down and fixed within 
 the alveolar border. Beginning at the lower parts, the roots 
 are again reabsorbed, along with the bony case of their 
 sockets, even before the term of functioning of the teeth is 
 completed. Radiographs showing partially reabsorbed roots 
 present very much the same appearance as those showing par- 
 tially developed ones. Apparently the reabsorption, or resorp- 
 tion, as it is often called, is incited by the pressure of the 
 developing permanent crowns below. At any rate, in normal 
 cases the process is sufficiently advanced by the time the per- 
 manent crowns are ready to appear to allow the temporary 
 tooth to be readily removed with very slight disturbance and 
 very little pain. 
 
 Evidently here are processes in which the different types 
 of cells in the peridental membrane described above, especially 
 the osteoclasts, play a very important part. Pedley and Har- 
 rison (63, p. 52) would credit the leucocytes with an impor- 
 tant part in the resorption process also. 
 
 Aside from the resorption of the temporary roots there 
 is also a process of resorption of the cancellated tissue that 
 surrounds the deeply embedded crowns, allowing for their 
 passage upward toward the surface. In case of the molars, 
 this includes the absorption of part of the cortical bone along 
 the borders of the jaws where there have been no preceding 
 temporary teeth. 
 
 The growth process by which the crowns emerge, making 
 additions to the roots while so doing, is not essentially dififer- 
 ent from that of the temporary ones already described. Build- 
 ing up of the roots seems to be a somewhat slower process 
 in this case, and their final cementing into place more per- 
 manent. This latter is rather an osseous change and will be 
 mentioned more in detail later, but it should be noted in this 
 connection that the order is always first tooth position, then 
 adjustment of foundation. This is true regardless of whether 
 the position assumed is normal or abnormal. The bony foun- 
 dation builds to the tooth, whatever may be the position it 
 assumes. 
 
 Again, difference in size, especially of incisors and canines, 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 295 
 
 is greater in the upper than in the lower jaw, so that read- 
 justment of the two columns of teeth to each other becomes 
 necessary. This is a fact of considerable importance for the 
 normal occlusion of the teeth of the permanent set. Whereas 
 the upper and lower teeth of corresponding names fit end to 
 end in the temporary set, in the permanent, the greater size 
 of the teeth in the upper column cause a backward shift along 
 the sides. The normal condition is therefore for each molar 
 and bicuspid to articulate, not with a single antagonist in the 
 other jaw. but with two teeth. 
 
 So much for the phenomena of dentition. The treatment 
 is inadequate, but possibly sufficient facts have been presented 
 in this brief review to indicate that the beginning of later 
 childhood is without a doubt a transitional period as regards 
 the teeth. Preceding this period time and material are both 
 devoted to the task of providing temporary structures and 
 preparing permanent crowns ; with the eruption of the first 
 molars at this period, however, is ushered in a series of re- 
 markable changes, every one of which is a step in the direc- 
 tion of dental maturity. Dr. Port (65) stated the case nicely 
 when he referred to this period of life as one at which, for the 
 dentist, the child becomes adult. The significance of these 
 phenomena becomes greater when they are associated with 
 others closely allied to them, and, in a way, depending upon 
 them. To some of these we shall next give attention. 
 
 Paralleling that of the teeth, there must occur a pro- 
 gressive developmeiit of the bony arches of the jaws that 
 form their foundations. Structurally, both jaws are complex, 
 and are in very close relationship to the other parts that' 
 make up the facial portion of the skull. Thus the upper jaw 
 forms the principal bone of the face, taking part in the 
 formation of the hard palate, the floor of the orbit and the 
 floor and lateral wall of the nasal cavity (71a, p. 50). Cryer 
 states (27, p. 31) that in a surgical operation removing 
 a right or left maxilla, " the inferior turbinate, portions of 
 the lachrymal, the palatal, the malar, and the ethmoid bones 
 will probably be removed with it." The lower jaw, on the 
 other hand, has the distinction of being the strongest and 
 thickest bone of the face. It is made up of the arched por- 
 tion known as the body, and the two ascending portions called 
 the rami, each of which has a flat, pointed projection at its 
 anterior edge for the attachment of muscles, and at the 
 posterior edge a convex process which hinges with the for- 
 ward portion of the temporal bone and allows for the move- 
 ments of the jaw. 
 
 Leaving aside for the moment the details of structure, we 
 
296 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 may generalize with regard to the arched portions of the 
 two jaws by saying that in either case, the essential features 
 are (i) a basal osseous arch composed of an outer shell 
 of cortical bone filled in with spongy, cancellated tissue, and 
 (2) an alveolar border consisting of two thin, hard, compact 
 plates, an inner and an outer, fitting close to the roots of the 
 teeth, between which is the looser cancellated portion sur- 
 rounding the roots of the teeth, and within which the more 
 compact, shell-like cases surrounding the roots are contained. 
 The whole arrangement of the jaws, both with reference to 
 each other and to the remaining portions of the face and 
 skull, is such as to withstand the strain of muscular con- 
 tractions in chewing, and at the same time protect against 
 forces from without, such as blows, by diffusing them in 
 a way to prevent their being transmitted to the brain case 
 
 As already intimated in a previous paragraph, the develop- 
 ment of the jaws begins very early in the embryo. Accord- 
 ing to Ouain's Anatomy (71), the order for the begin- 
 ning of calcification of the bones is : first, the clavicle ; second, 
 the inferior maxillary (mandible) ; and third, the superior 
 maxillary (maxillae). The Text-Book of Cunningham places 
 the beginning at about sixth or seventh week of foetal life 
 (28, pp. 144-5). Witzel would seem to agree as to time, 
 since he published a plate (98, plate 3), showing calcification 
 of the mandible in a foetus of six weeks. 
 
 In case of either jaw, calcification begins in a number of 
 centers which later unite. The final completion of this 
 process does not occur in the mandible until some time after 
 birth (usually during the course of the first year), when the 
 two halves unite. 
 
 Extended and accurate investigations of the growth of the 
 jaws from this time until the beginning of second dentition 
 are very few. On a few points, however, authorities seem 
 to be in agreement, so that we may characterize in a general 
 way the principal developmental features of this period. 
 Naturally we turn first to longitudinal growth. After the 
 completion of the first year, or at least after the completion 
 of first dentition, there is little or no growth in those 
 portions of the arches containing the temporary teeth. If 
 any occurs at all it is very small in amount. With the 
 appearance of the teeth there is a lengthening of the ascend- 
 ing portions of the lower jaw, thus making room for the 
 teeth and their alveolar borders. The relationship of this 
 development to the order of the appearance of the teeth con- 
 stitutes one of the nice adjustments of nature, as Pedley and 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 297 
 
 Harrison (63, p. 42) have pointed out. In general, the order 
 of tooth appearance is from front to back. By this means 
 time is gained for the growth in the length of these branches, 
 while at the same time there is not left an " open bite " in 
 the middle of the arch. 
 
 As regards internal structure, arrangement of the fine bony- 
 particles or trabeculae remains for the most part regular, 
 though there are some changes having to do with strengthen- 
 ing the jaws and adjustment to the developing crowns of 
 the permanent teeth (98). 
 
 The rapid growth of the jaws at the time of second den- 
 tition is a well known fact. It is evident in the changed pro- 
 portions of both jaws and face, with the resulting change in 
 facial expression, and is also inferred from the fact that the 
 second set contains a greater number and, to all appearances, 
 larger teeth. 
 
 Growth is largely dependent upon the development of the 
 teeth, and is therefore not uniform in all parts of the jaws. 
 A number of investigators have attempted to answer the 
 question as to whether there is any longitudinal growth in 
 those portions of the arches bearing the temporary or decidu- 
 ous teeth at the time of second dentition. Zsigmondi (106) 
 reports the results from a long series of these investigations, 
 and in addition describes an investigation of his own, in 
 which, by the use of plaster casts, he employed a new method, 
 obtaining records of the same jaws at different stages of their 
 development. 
 
 Difficulties of method are everywhere apparent, not the 
 least of which is that the measurements give a record, not of 
 the differences in the basic part of the jaw, but rather of those 
 in the dental arch or the alveolar border only. Re- 
 ferring in part to the results reported by Zsigmondi, in part 
 to the original articles themselves, and comparing carefully, 
 it seems only safe to say that there is not conclusive evidence 
 that any longitudinal growth occurs in that portion of the 
 alveolar arch lying between the first permanent molars during 
 the period of second dentition. Since this is true, there is 
 likewise no evidence to show that longitudinal growth occurs 
 in the basic portions. 
 
 That the dental arch in the upper jaw becomes larger is 
 not disputed, but attention is called to the fact that here the 
 teeth with their alveolar border slant outward, while in the 
 lower jaw they slant rather inward, so that the question of 
 longitudinal growth of the basic part of the jaw is still left 
 unanswered. Besides, the question as to the presence or 
 absence of growth is most often raised with reference to the 
 
298 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 lower jaw, the result being that most investigations have had 
 to do with it, and have secured data pointing more or less to 
 conflicting conclusions. However, while conclusive proof is 
 lacking, there is not general agreement that growth does not 
 occur. More recently Cryer (27, p. 14), who bases his con- 
 clusion upon the study of the anatomical arrangement of the 
 inner structure of the mandible, has declared in favor of 
 growth. Angle also seems to hold this view (i, p. 91). 
 
 While growth in the " deciduous arch " is still questioned, 
 on one other point authorities are agreed. While the perma- 
 nent incisors and canines require a greater amount of space 
 than their predecessors, the greater requirement is partially 
 compensated for by a lesser space requirement on the part of 
 the permanent bicuspids, which are smaller. The actual need 
 for growth in these portions of the jaws is therefore much 
 less than it would be were the permanent teeth uniformly 
 larger than their predecessors. The compensation is much 
 less nearly complete in the upper than in the lower jaw, thus 
 giving a second partial explanation of the fact that the upper 
 permanent dental arch is apparently larger, while the increase 
 in the size of the lower is questioned. 
 
 Let us turn attention next to the posterior portions of the 
 jaws. It should be noted first that authorities generally agree 
 that growth of these portions is dependent upon the develop- 
 ment and eruption of the molar crowns. In either jaw the 
 growth must be sufficient to make room for the three molar 
 teeth on each side. The development of these crowns seems 
 to be the signal for growth of this portion of the jaws. This 
 is especially noticeable in the lower jaw. " The cutting of the 
 first molar tooth and the formation of the crown of the second 
 cause in the following years of life a strong longitudinal growth 
 of the maxillary body, . . ." writes Witzel (98, p. 59). 
 
 As in case of the anterior portions of the arches, there is 
 much disagreement as to the exact process by which the 
 increase is brought about. Some authorities, such as Cryer, 
 hold that growth is general along the jaw and that each erupt- 
 ing crown pushes the ones anterior to it forward {2'j, p. 14). 
 Others take the position that the growth is only at the ex- 
 treme posterior portion, and that much of it is due to absorp- 
 tion and deposition of the bone substance. Humphry quotes 
 Tomes (48, p. 3, footnote) to the effect that these processes 
 are shown by the examination of the histological structures 
 at time of growth. Humphry also tested the growth. in the 
 lower jaws of young pigs by piercing the ascending portions 
 and tying wires about the anterior and posterior borders. 
 Later, when the pigs were killed, the wires at the anterior 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 299 
 
 border were found to have either become loosened or dropped 
 out, showing that resorption of bone had taken place, while 
 the wires at the posterior borders were found in deeply in- 
 dented notches formed from the progressive deposition of 
 bone above and below them (48, p. 3 ff.). 
 
 Besides the horizontal growth in the arches of the jaws, 
 adjustment to the greater space required between them to 
 accommodate the larger permanent set is accomplished by 
 means of a growth in the length of the ascending portions, 
 or rami, of the mandible. With the longitudinal growth down- 
 ward and backward the angle of the jaw becomes more acute, 
 approaching a right angle, though this is apparently due in 
 part also to the development of the teeth and alveolar border. 
 Finally there may be mentioned a group of changes of inner 
 structure. Rearrangement of the teeth brings about a neces- 
 sary redistribution of their neural and vascular supply. In- 
 crease of masticatory surface, accompanied by a growth in 
 strength of the muscles used in mastication, causes a strength- 
 ening of the basic portions of the jaws ; lastly, as a result of 
 rearrangement of the lines along which the forces are exerted 
 in mastication, as well as of growth, there occurs a new 
 arrangement of the bony trabeculae within the jaws to adapt 
 to the new conditions. 
 
 In addition to that of the jaws, some other developmental 
 factors appear to be very closely related to dentition. Pos- 
 sibly most prominent among these is the growth of the face, 
 due in part, of course, to the increasing size of the jaws. 
 West found in his Worcester investigation (97) that there 
 are pretty distinct periods in the development of the female 
 face, the first division point being at about the seventh year. 
 Transitions from one type to the one next following seemed 
 to be rather abrupt. Rose found from head and face meas- 
 urements of more than 45,000 children of different nation- 
 alities that the face lengthens much more rapidly during the 
 time of change of teeth than it does in later years (75, p. 
 711). Facial index increased in length some three and one- 
 half degrees between ages seven and thirteen (75, Table 5, 
 p. 706). 
 
 Greater masticatory ability seems to be accompanied by 
 an inner psychic change. Bell (7) found in his study of the 
 psychology of foods that taste becomes " mentalized " at 
 about the age of seven as it has never been before. He speaks 
 of this as a stage of " teasing to taste." He found also a 
 tendency to make every possible mixture at this period, and 
 an increased interest in medicine, with a tendency to taste it. 
 Dr. Wright (100) finds that periods of enlargement of the 
 
300 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 tonsils without inflammation coincide with the times when the 
 groups of molars are erupting. He finds that after giving 
 the tonsil prophylactic treatment, when necessary, and wait- 
 ing for the tooth to erupt, the return of the enlarged tonsil 
 to the normal will follow in a high percentage of cases. Dis- 
 appearance of abnormal tonsilar conditions also follows the 
 treatment of carious teeth in many cases. 
 
 He contends that tonsilar enlargement without infection is 
 but an expression of the normal functioning of lymphoid 
 tissues in that region, especially active at times of tooth erup- 
 tion because of additional requirements in the way of caring 
 for waste material which results from rapid bone resorption, 
 etc. The second period when this condition is liable to be 
 present coincides with that for the eruption of the first per- 
 manent molars, and therefore in a general way also with 
 the period of life in which we are here especially interested. 
 
 One hesitates to leave this topic without dwelling for em- 
 phasis upon the importance of dentition for development at 
 the beginning school age. Referring to the relationship of 
 the teeth to nutrition, Crampton (26, p. 354) has spoken of 
 tooth appearance as " the indication of successful growth and 
 the earnest of further preparation for growth." 
 
 Again, importance of dentition stands out clearly when one 
 takes into account the number of conditions by which its 
 progress is affected. The great variation in time of appear- 
 ance has been noted. There are many explanations for this. 
 Early or late dentition have often been charged up to here- 
 dity, and facts regarding the first set, such as those presented 
 by Holt (45, p. 28) and Rosenhaupt (79) would seem to 
 indicate that there is reason for doing so. Rose has found 
 that permanent teeth appear earlier among Swedish than 
 among German children, thus showing the influence of race 
 {77). He would also credit the chewing of coarser foods 
 with some influence in this case. Again, in the same investi- 
 gation. Rose found dentition earlier in children of the higher 
 schools than in those of the Volkschule, and earlier in chil- 
 dren of city than those of rural communities. This is ex- 
 plained as due to differences of nourishment, and to some 
 extent to better racial selection also. The importance ot 
 nourishment while the crowns are developing, therefore dur- 
 ing the first five or six years of life, is emphasized by many 
 authorities. Bell (7), without giving his authority, states 
 that it is reported southern children erupt their teeth earlier 
 than northern. Finally, it is recognized that diseases influ- 
 ence dentition. Holt (45, p. 28) says syphilitic children are 
 prone to early dentition, in which case rapid decay is likely 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 3OI 
 
 to follow. Rickets is usually credited with having a retarding 
 influence. Rose tested this {jy, p. 569) by comparison 
 of children with normal teeth and those with hypoplasia, or 
 deficiency of enamel. Among 10,020 children, he found a 
 slightly later eruption time in the latter group. Latest denti- 
 tion is seen in cretinism (45, p. 29). 
 
 The complexity of the problem is evident. Berten says 
 (8, p. 267) that whoever has given only slight attention to the 
 eruption of the teeth will have found that the time varies 
 with race, climate, sex, constitution and disease, and in part 
 to sports of nature. 
 
 Composition of the tooth crowns is apparently affected by 
 many of the same influences, especially conditions of nourish- 
 ment and disease. Lack of lime in food and water may cause 
 poor dental structures, with deficient enamel, and therefore 
 poor protection. Dr. Burnham (18, pp. 296-7) reports a 
 number of investigations indicating that regions poor in lime 
 show a high percentage of carious teeth. Pedley and Harri- 
 son (63, p. 47) emphasize the use of the teeth as beneficial, 
 in that it increases blood supply and thus brings more nour- 
 ishment. The disease of rickets apparently deprives the teeth 
 of sufficient lime salts, being accompanied usually by hypo- 
 plasia, a condition in which the enamel is deficient. 
 
 Finally, we may cite the response of the organism to dental 
 conditions, as evidence of the importance of the latter at 
 the school age. Among others, Jessen (50, p. i) quotes these 
 conclusions from Rose's " Zahnvcrdcrbniss und Zcnsur " 
 (Deuts. Monats. f. Zahnheilk., 1904) : 
 
 " I. The physical development of school children is greatly 
 influenced by defective teeth. 2. The poorer the physical 
 development, the poorer also is the mental power (" Spann- 
 kraft ") of the children. 5. The poorer the teeth, the worse 
 are, on the average, the grades of the children." In his arti- 
 cle on the " Hygiene of the Teeth " Dr. Burnham writes 
 (18, p, 298) : " Of the various disorders frequently caused or 
 aggravated by decaying teeth are not only enlarged glands 
 of the neck, headache, neuralgia, earache, but indigestion, 
 heart trouble, irritation of the nervous system, epilepsy, and 
 perhaps chorea and other neuroses." 
 
 Preservation of a full set of sound temporary teeth and 
 regulation of their positions when necessary are requisites for 
 proper development of both teeth and jaws at time of second 
 dentition. Premature extraction, too long retention, caries or 
 malocclusion in case of the temporary set may be the cause 
 of improper development of the jaws with impaction or with 
 abnormalities in position of the permanent crowns. 
 
302 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 Some experiments upon animals would seem to indicate 
 the dependence of proper development in jaws and face upon 
 a full set of functioning teeth. Walkhoff (92) cut one tem- 
 poral muscle of a dog four weeks old, compelling him to 
 chew only on the other side. Within three months a differ- 
 ence between the development of the two sides of the jaws 
 could be detected, and at the end of a year the functioning 
 side was much larger and stronger than the other. Dr. Baker 
 (5) performed a similar experiment by grinding the teeth of 
 young rabbits, preventing occlusion in one-half of the jaw. 
 Not only the jaws, but the parts of the skull to which the 
 muscles of mastication were attached showed the effects by 
 their greater size and strength on the functioning side. The 
 unused sides remained undeveloped. The prepared skulls 
 showed a distinct asymmetry. 
 
 While emphasizing the point that the presence of all the 
 teeth is essential, Dr. Angle has stated (i, p. 17) that, "in 
 function and influence some are of greater importance than 
 others, the most important of all being the first permanent 
 molars." He explains further that this is true because, /. 
 they are largest and firmest in their attachment, 2. they have 
 the most important location in the arches, j. their length de- 
 termines the separation of jaws and length of bite, thus 
 contributing to facial proportions, 4. they are first in position 
 and influence other tooth positions, and 5. they are most 
 constant in time of eruption and in the assumption of normal 
 positions. 
 
 From all these facts, and especially in view of the close 
 relationship of tooth development and nutrition, it must fol- 
 low that dentition is a matter of prime importance in the 
 early years. It would perhaps not be too much to say that, 
 from among all the factors which may and should be con- 
 sidered for such a purpose, dentition is the best single indi- 
 cator of the stage of physical development which a child 
 has reached at any time during these early years, or, as some 
 would state it, of physiological age. 
 
 Having dwelt somewhat at length upon this very import- 
 ant phase of development, we may bring together the prin- 
 cipal points in this brief summary: 
 
 I. The development of the teeth and the parts closely re- 
 lated to them, particularly the jaws, give evidences of a 
 transitional stage at about the time of the beginning school 
 years. The loss of the temporary and the eruption of the 
 second, permanent, more numerous, adult-sized set of teeth 
 is begun at this time, producing a new, different method of 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 303 
 
 articulation. This is accompanied by rapid growth in certain 
 portions of the jaws, with considerable rearrangement of the 
 inner structures, resorption and deposition of bone sub- 
 stance, etc. 
 
 2. Normally, the transition is begun by the eruption of 
 the first permanent molars, at about the seventh year, after 
 which temporary tooth groups are replaced by the perma- 
 ment in the order ; inner incisors, outer incisors, first bicus- 
 pids, with canines and second bicuspids following in variable 
 order, and followed in turn by second molars. Wisdom teeth 
 come much later. 
 
 J. The time of tooth appearance in dififerent children 
 shows considerable variability, beginning with the early years, 
 clearly evident at the time the first molars erupt, and increas- 
 ing rapidly with the progress of dentition thereafter. 
 
 4. Sex differences are apparent in the eruption time of the 
 permanent teeth, those of the girls appearing earlier, on the 
 average, than those of boys. Here also, the difference in- 
 creases with the progress of dentition. 
 
 5. The vital significance of dentition as a factor in develop- 
 ment is shown by its close relationship to nutrition, by its 
 close relationship to the growth of other parts, such as jaws, 
 face and skull; by the many influences, such as nutrition, 
 race, sex, etc., by which it is influenced ; and by the reaction 
 of the organism to dental conditions as shown in physical 
 development, mental power, and the like. 
 
 6. These things being true, it follows that the state of 
 advancement reached in dentition is a good indication of the 
 stage of progress which a child has reached in his total physi- 
 cal development. The transitional features described there- 
 fore take on an additional significance. 
 
 Grozvth of the Skull. — The period of life under considera- 
 tion is apparently transitional as regards the growth of the 
 skull. In Ouain's Anatomy (70, p. 82) we read: 
 
 "The skull grows rapidly during the first seven years of life. By 
 that time, certain parts, including the circumference of the occipital 
 foramen, the body of the sphenoid, the cribriform plate, and the petrous 
 division of the temporal have attained their definitive size. The 
 other regions also increase but little until the approach of puberty, 
 when a second period of active growth begins, affecting especially 
 the face and frontal portion of the cranium, with which is associated 
 the expansion of the frontal and other air sinuses." 
 
 These facts become the more interesting when we recall 
 that cessation of growth in these portions occurs at just 
 about the same time as the beginning of more rapid growth 
 
304 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 in parts of the face, as previously described, and as demon- 
 strated by West (97) and by Rose (75). 
 
 Grozvth of the Brain. — Data with regard to the size of 
 the normal brain at different stages of development are 
 meagre. Aside from this, there are many difficulties which 
 arise from unavoidable inaccuracies in methods of sectioning 
 and preparing specimens, and also from the fact that indi- 
 vidual variations are great. Averages of brain weights are 
 therefore of doubtful value. 
 
 It is generally agreed that the early years are years of very 
 rapid gain in brain weight. There are no data, however, to 
 show beyond question that a distinct nodality occurs at the 
 beginning of the later childhood period. Curves for brain 
 weights at different ages can only be said to suggest that 
 this is true. Pfister is reported to have found that the brain 
 weight at the end of the sixth year is not infrequently equal 
 to that of the adult (71, p. 342). This is also observable in 
 the tabulation of brain weights collected by Vierordt (90, 
 pp. 36-37). Donaldson, in his study of the brain was led to 
 conclude, largely on the basis of the curve for brain weights 
 collected by Vierordt, that, '\ . . .By the seventh year the 
 encephalon has reached approximately its full weight, the sub- 
 sequent increase being comparatively small . . ." (30, p. 104). 
 We may say, therefore, not that brain weights show, but 
 rather that they suggest a transition from rapid gain in gross 
 weight to a different mode of development from the 'school 
 age onward. This suggestion is strengthened by the fact that 
 the rapid growth of the skull ceases at about the same time. 
 Again, psychological investigations seem to show for this 
 period a rapid development of muscular control, and the like, 
 suggesting development along the line of better mner organi- 
 zation rather than by means of mere addition of material. 
 
 Development of the Eye. — Embryonically, the eye develops 
 as a specialized portion of the brain. It is therefore interest- 
 ing to note that there is some indication of its having attained 
 approximately adult size at the same time with the brain. 
 Stratz (86, a) has noted the relatively large size of the eye 
 as compared to the small face of the child in the early years, 
 due in part, of course, to the rather late development of the 
 facial portions. The statement of Merkel would indicate that 
 there is a further possible explanation. Without giving data, 
 he says (55, Vol. i, p. 256) : " In connection with the de- 
 velopment of the whole eye-ball, it occurs that the horizontal 
 breadth of the iris is already reached in the sixth year of 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 305 
 
 life." Parallel to this fact — provided data may be found to 
 support it as fact, — there occurs at about the school en- 
 trance age a transition in power to use the eye and also to 
 control its movements. 
 
 An important factor is the power to employ binocular 
 vision. In this is involved the ability to " fuse " the slightly 
 different images from the two eyes and interpret them as one 
 object. This begins in the first few weeks of life, and accord- 
 ing to Miss Sayer (82), is usually complete about the sixth 
 to ninth year of life. In learning to overcome the difficulty, 
 the child has a tendency to " squint," but overcomes it after 
 learning to use the eyes properly. From defects of vision, 
 however, he may develop a permanent " squint." This is 
 easily cured by means of the amblioscope if taken before the 
 age of six, but otherwise it develops, as does stammering, 
 into a pathological condition that is very difficult to cure. The 
 statement of Dr. Cornell (23, p. 243) with regard to this con- 
 dition is in agreement with that of Miss Sayer. Again, while 
 largely dependent upon central brain structure, there should 
 be mentioned in this connection the lack of power to retain 
 visual imagery in case of those persons who become blinded 
 earlier than the fifth to seventh year (49). 
 
 Here, again, phenomena which apparently involve both 
 visual organs and visual brain centers would seem to suggest 
 strongly that the closing years of earlier childhood constitute 
 a time of change, both with respect to structural development 
 and functioning. 
 
 Larynx and Voice. — Vocal organs and vocal powers give 
 evidence of a nodality of development at beginning of the 
 later childhood period. Of the larynx Barth (6, p. 86) 
 writes : 
 
 "In the first two years of life the growth of the larynx is very 
 slight. With the active use of speech in the following years the 
 growth progresses more rapidly. It appears, however, to be limited 
 more to the muscles that move the vocal cords. From the sixth to 
 the fourteenth year of life the growth of the larynx is almost sta- 
 tionary again, or, if it is present, is very slightly noticeable, strikingly 
 and disproportionately less noticeable than the growth of the remain- 
 ing organs of the body." 
 
 Cunningham's Anatomy makes a similar statement (28, p. 
 972) : 
 
 " In the newly born child, the larynx, in comparison with the rest 
 
 of the body, is somewhat large ( ) and it continues to grow 
 
 slowly and uniformly up to the sixth year of childhood. At this 
 period there is a cessation of growth, which persists until puberty 
 is reached, and then a stage of active growth supervenes." 
 
3o6 
 
 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 Gutzmann's diagram of the averages for normal children 
 of both sexes up to the age of fifteen {2,7, p. 51) indicates 
 a rapid widening of the range of voice, beginning at the age 
 of seven. Differentiation of the sexes is also shown at this 
 time, the range for girls being wider than that for boys from 
 this age onward. The diagram is as follows. 
 
 * 
 
 Age: O 
 
 i 
 
 1-2 
 
 i 
 
 3-5 
 
 St 
 
 ^^-6 
 
 »--t5»--?5 
 
 Z/^ 
 
 9 
 
 10 
 
 12 •: 
 
 11 
 
 zn^rrz 
 
 13 
 
 14^ 
 
 N euro -muscular Control. — Differences in the power to con- 
 trol bodily organs indicate a transitional period at six or seven 
 years. Speech development also gives such an indication. 
 Apparently this period marks a stage of completion in speech 
 coordinations, since infantile babble, if continued after this 
 time, is usually considered pathological (22). Control of 
 visual organs, as previously described, gives a similar indica- 
 tion. Unless corrected before this period, " squint " becomes 
 permanent. Vocal power, on the other hand, as indicated by 
 the range within which pure tones can be sung, shows a rapid 
 increase from this time on (37). The two sexes also show a 
 difference in breadth of range from this period onward, at 
 least up to the pubescent years. Bryan's tests of precision in 
 finger movements (14) showed that in both the "up" and 
 " down " writing movements almost half the gain between the 
 ages of si'x and sixteen was made between the ages of six 
 and eight years. Halleck has stated, " The vital time for 
 motor training is before the age of eight " (40, p. 835). 
 
 Pathological Conditions. — Some pathological conditions 
 may be grouped together to show a relationship to this period 
 of life. Stuttering, which differs somewhat from the infantile 
 babble previously referred to, has been shown to be more 
 prevalent at seven to eight years of age. Conradi (22) be- 
 lieves there is a causal connection between this and dentition. 
 Jastrow (49) found among some 200 persons in institutions 
 for the blind that those who had lost the power of sight 
 previous to the age five to seven years did not retain 
 visual imagery in their dreams in after years. Those 
 blinded later than the age of seven, on the other hand, did 
 retain visual imagery. Clouston, in his " Neuroses of Devel- 
 opment " (21^), long ago observed that the age of seven or 
 eight is a time when many neuropathic conditions are liable to 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 307 
 
 break out. In this connection again should be mentioned that 
 " squint," also infantile babble are but natural when occur- 
 ring previous to this age, while if lasting longer they seem 
 to be so deeply grounded as to constitute pathological condi- 
 tions. 
 
 These are by no means proofs, nor is the list complete, yet 
 the suggestion is strong that after this period of life the 
 physical organization is somewhat different from that which 
 precedes. 
 
 Mental Development. — Adequate treatment of this topic 
 would make this paper far too voluminous. We can touch 
 it but briefly, stopping only for some general statements, 
 chiefly concerning its relationship to our problem. 
 
 Early years have long been recognized as a time of sensory 
 training, shorter attention span, shorter memory span, etc., 
 but no sharp transition has been clearly shown. Mental tests 
 are not yet sufficiently refined to grade accurately small steps 
 of advancement. Individual variations are too great. 
 
 Though evidence to the contrary may be found, that there is 
 a correlation of mental and physical development during 
 school life has been indicated in a general way by such in- 
 vestigations as those of Porter (67), Smedley (84), Ouirsfeld 
 (72) and Crampton (26). Methods of grading mental ad- 
 vancement with reference to stage of physical development 
 rather than chronological age at this period, however, have 
 yet to be worked out. There is needed here much further 
 investigation, on the individual plan, for the purpose of find- 
 ing the degree of mental advancement that may be expected 
 in a pupil of either sex who has reached a given stage in his 
 or her physical development. The question arises as to 
 whether such investigation might not show a characteristic 
 of mental development corresponding to the transitions in the 
 physical. Facts of brain growth, neuro-muscular control and 
 the like suggest that this might be true. 
 
 Summary. — By way of summary we may generalize with 
 regard to the preceding facts bearing on the question of 
 nodality at the school entrance period as follows : 
 
 1. Disturbances of growth in height and weight have been 
 noticed at this period but their nature is imperfectly under- 
 stood. 
 
 2. Dentition shows distinctly a transition at this period, and 
 because of close relationship to other phases of development 
 suggests strongly that it is general in its nature. 
 
 5. Both skull and brain show a change in growth rate at 
 about this time of life. 
 
308 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 4. Growth of larynx and development of voice range indi- 
 cate transition in these years. 
 
 5. The statements of some authorities with regard to 
 growth of the eye and control of its movements suggest nodal- 
 ity in its development at this age. 
 
 6. Phenomena of muscular control, also facts regarding 
 pathological conditions, give evidence of a change in physical 
 organization after this period. 
 
 7. A clear-cut mental transition has not been shown, but 
 tests have not related mental to physical stages of advance- 
 ment. General correlations of mental and physical powers 
 and facts regarding brain growth and muscular control sug- 
 gest that tests relating these phases would indicate a transition 
 here also. 
 
 On the basis of these facts it appears that the question 
 regarding a transitional stage of development at the begin- 
 ning of later childhood should be answered in the affirmative. 
 The time at which these phenomena occur is of course only ap- 
 proximate, being based upon averages in most cases. The 
 degree to which these changes are interdependent is also un- 
 known. Whether they are all secondary to one basal, radi- 
 cal change, as for example dentition, or whether they repre- 
 sent a more general reorganization to which each is related 
 cannot be stated. It seems probable that the latter is the 
 case, although it is a problem that must be left open. It is 
 certainly one that merits further investigation. 
 
 Comparative Development of the Sexes. 
 
 If, now, we give attention to the question of the compara- 
 tive development of the two sexes at the school age, we are 
 led to the consideration of a number of groups of data simi- 
 lar to those of the preceding section. 
 
 Occasional statements with regard to differences of the 
 sexes at this early period of life have been made. Tyler (88, 
 p. 139) thought girls of six begin to show signs of the precocity 
 which characterizes later development. Rose mentioned dif- 
 ferences of sex at time of second dentition {yy, p. 555). 
 Stratz (86 a) thought sex differences in bodily form first be- 
 come evident at the age of six, after which girls show more 
 grace of curve, boys more of angularity and strength. For 
 the most part, however, it has been taken for granted that the 
 development of the two sexes is the same until some time 
 after this period of life is passed. 
 
 In an article published at a time w^hen this study was near 
 completion, Dr. Boas (9) made some comparisons of develop- 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 309 
 
 ment, particularly in dentition and ossification of the skeleton, 
 which showed a greater advancement on the part of girls at 
 this period. As he has stated, however, the number of data 
 was limited, and the study rather of a preliminary nature. 
 While his conclusions are partially correct, it may still be 
 said that no extended comparison of facts has been made to 
 decide the matter of comparative advancement of the physical 
 development in the sexes at the beginning school period of 
 life. It is our purpose here to attempt such a comparison. 
 
 Height. — Comparison of absolute height at any given time 
 in the early years does not show any cause for a differentia- 
 tion of the sexes. The same is true of comparisons where 
 added increments are reckoned as percentages of previously 
 attained height. A question arises, however, as to whether 
 this is an adequate method for getting at the actual attainment 
 for a given age. At maturity the male is considerably larger 
 than the female. Nor do they reach maturity at- the same 
 time. If, then, our final comparison is to be one of unequals, 
 comparisons previous to that time should not be on a basis 
 of absolute equality, else erroneous conclusions will result. 
 What we should ask is not, " Does the height of the boy equal 
 that of the girl at a given age?" Rather should the question 
 be, " What are the percentages of final, adult height reached 
 in the two cases at a given age? " 
 
 Comparison by such a method has been attempted. In a 
 preliminary reckoning, taking Dr. Boas' averages for children 
 of larger American cities, as given by Burk (17, Table H.), 
 the average height of boys at 6.5 years is 69.88% of the ave- 
 rage at 15.5 years, while of girls it is 70.35%. 
 
 Results thus computated, on the basis of data from a num- 
 ber of extended investigations, are given in Table D. Except 
 in the last three cases, where the references are indicated, these 
 are on the basis of the corrected averages given in Burk's 
 "Table A" (17). These last three are not the corrected 
 averages. Since adult heights could not be secured from the 
 tables, the average at 17.5 years has been chosen as repre- 
 senting post-pubescent height. 
 
 Since girls are nearer adult height at the age of 17.5 years 
 than boys of equal age, it is evident that they are at a dis- 
 advantage in these comparisons. Yet in every case the indi- 
 cation is that they have attained a greater percentage of their 
 post-pubescent height at the age of 6.5 years than boys of the 
 same age. Comparisons with averages of adult height would 
 be more accurate, but it is plain that they would also show 
 greater differences of the percentages, thus making the girls 
 appear still more advanced, relatively. 
 
3IO 
 
 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 TABLE D. 
 
 Percentage of Post-pubescent Height Attained at 
 School Entrance Period. 
 
 Investigator 
 
 A 
 
 
 Com- 
 pared 
 to 
 
 Percentage for 
 
 "■a^ 
 
 Boys 
 
 Girls 
 
 Bowditch (Boston) 
 
 Porter (St. Louis) 
 
 Peckham (Milwaukee) . . 
 
 (Oakland) 
 
 West (Worcester). . . 
 
 Gilbert (New Haven) . 
 
 Gilbert (Iowa) 
 
 Key (Sweden) 
 
 Anth. Com'n . . (England) .... 
 
 Quetelet (Belgium) . . . . 
 
 Pagliani (Turin) 
 
 Hertel (Denmark) . . . 
 
 ' Smedley (Chicago) 
 
 ^ Lange (Germany) .... 
 
 ^Bobbitt (Phil. Is.) 
 
 6-5 
 
 < 
 
 yrs. 
 5 
 
 i7-5/rs. 
 
 16.5 yrs. 
 17.5 yrs. 
 
 17 
 
 66.08 
 66. 
 66 . 10 
 58.20 
 
 66 .90 
 
 67 .06 
 65-98 
 69 .40 
 66 .46 
 65.60 
 64 .60 
 68.11 
 66 .99 
 60 .38 
 72.89 
 
 70.03 
 67 .60 
 
 69 .60 
 
 70 .80 
 69.70 
 70.70 
 69.14 
 70 .60 
 68.60 
 66.66 
 65 .90 
 70.40 
 70.29 
 69.31 
 7«-33 
 
 iSee (84). 
 
 See (90, pp. lo-i i). 
 
 See (12). 
 
 As a matter of interest, comparison of average heights of 
 feeble-minded in nineteen institutions, as given by Goddard 
 (35), was made, using ages six and twenty. Percentage of 
 twenty-year height reached at the earHer age was, for males, 
 66.76, for females, 68.82. 
 
 Weight. — As in case of height, data for the average at- 
 tainments of weight have not indicated any differences in the 
 sexes preceding the pubescent period of growth. Both Porter 
 (66) and Burk (17) noted a slight superiority of boys during 
 the first few school years. Applying the method of compari- 
 sons just used in case of height, however, and using again 
 the corrected averages from Burk's collection of data (17, 
 Table E), we obtain the results presented in Table E. 
 
 While there is slight lack of uniformity in these results, in 
 general it holds true that there is shown a superiority of girls 
 in the amount of post-pubescent weight attained at the ear- 
 lier age. 
 
 In case of both height and weight, therefore, the relative 
 attainment of girls is shown to be slightly superior to that of 
 boys at about the age of 6.5 years. 
 
 Skeletal Development. — Radiographic studies have shown 
 that ossification of the skeleton proceeds more rapidly in girls 
 than in boys during the early years. This is especially notice- 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 3II 
 
 TABLE E. 
 
 Percentage of Post-pubescent Weight Attained at 
 School Entrance Period. 
 
 
 Age 
 
 Com- 
 pared 
 to 
 
 Percentage for 
 
 
 Boys 
 
 Girls 
 
 Bowditch (Boston) 
 
 Porter (St. Louis) .... 
 
 (Oakland). .. . 
 
 Peckham (Milwaukee) . . 
 
 West (Worcester). . . 
 
 Gilbert (New Haven) . 
 
 Gilbert (Iowa) 
 
 Key (Sweden) 
 
 Hertel (Denmark) . . . 
 
 Erismann (Moskow) .... 
 
 Pagliani (Turin) 
 
 Anth. Com'n . .(England) .... 
 _Misawa (Japan) 
 
 6.5 yrs. 
 
 7-5 yrs. 
 6.5 yrs. 
 
 17.5 yrs. 
 
 16.5 yrs. 
 ly.S^^yrs. 
 
 35-45 
 35-58 
 36.14 
 
 34-35 
 34-64 
 36.00 
 32.82 
 39-60 
 39-64 
 34.83 
 31.66 
 
 33-91 
 34-61 
 
 37-48 
 35-94 
 38-99 
 38.04 
 38.00 
 38.96 
 34-38 
 39-53 
 39-23 
 42.37 
 34.57 
 36.10 
 
 37-14 
 
 able in the carpal bones of the wrists and the epiphyses of the 
 joints. Dr. Rotch makes this statement, but gives few data 
 (80, p. 416). Pryor's studies, however, give data by which 
 this seems to be clearly indicated (69), though their number is 
 limited. He quotes a conclusion from a previous paper by 
 himself as follows (69, p. 3) : " The bones of the female os- 
 sify in advance of the male. This is measured first by days, 
 then months, then years." 
 
 Choosing the trapesoid bone of the wrist as an example, 
 we may note that he finds its time for ossification to be be- 
 tween the fourth and fifth year in the female, between the 
 fifth and sixth year in the male (69). 
 
 Dentition. — Differences in the time of eruption of the 
 teeth should again be emphasized at this point. Greater ra- 
 pidity in the eruption of the teeth of girls was shown in case 
 of each of the investigations referred to, beginning with the 
 first molars and continuing up to the time of appearance of 
 canines and second molars. The difference is by no means 
 as great, however, at the first molar stage as Boas found 
 (9, p. 815) from his limited collection of data. Rose's 
 averages show the difference in time of first molars to be as 
 small as one to two months. It should be emphasized again 
 that the difference in eruption time of the teeth grows pro- 
 gressively greater with advancement of dentition. By the time 
 the second molars appear, the difference in time between the 
 
312 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 two sexes approximates the difference in time between their 
 attainment of pubescence. 
 
 Groivth of the Brain. — The limited number of data ren- 
 ders comparisons of brain weights for the two sexes very 
 difficuh. Only general indications may be noted. Curves 
 constructed on the basis of Vierordt's data (30, p. 105) seem 
 to indicate an earlier attainment of adult brain weight in 
 females. This can be said to be little more than an indica- 
 tion, however, on account of the difficulties of method for- 
 merly referred to and the great variability in brain weights. 
 
 N euro-Muscular Development. — The probability indicated 
 by brain weights is in correspondence with that indicated by 
 facts of muscular control. Hancock found in his swaying 
 tests that girls were more steady in the early years than boys 
 (41). Automatograph tests showed better lateral control of 
 the arm, and tremograph tests showed greater control of fin- 
 ger movements among girls also. Observations of less defi- 
 nite tests again showed the superiority of girls in control of 
 movements. 
 
 Bryan's tapping tests seem to be alone in their showing 
 of better control on the part of boys. He says (14, p. 173) 
 that differences are slight, but that within the narrow limits 
 indicated there is a slight superiority of boys over girls. 
 
 Strength tests were compared by Burk (17) by a method 
 similar to the one we have used for height and weight. From 
 the data of Porter, Roberts and Gilbert he found percentages 
 indicating that boys had attained 1/5 their sixteen-year 
 strength at the age of six years, 1/4 from six to eleven. 
 Girls had gained a greater percentage at six, and from this 
 on their acquirement was m.ore rapid than that of boys. 
 
 Gesell noted that boys as a class show greater tendency 
 toward uncoordinated writing as early as the first grade and 
 up through the high school (33). 
 
 In this connection should be mentioned again that, begin- 
 ning with the age of seven, the range within which pure tones 
 can be sung is wider for girls than for boys. 
 
 Stuttering, which begins most often in the early years, has 
 been found in a number of investigations to be more preva- 
 lent among boys (22, p. 361). Various ratios for the sexes 
 have been reported. Conradi reports also that girls learn to 
 talk earlier than boys (22, p. 363). 
 
 With the exception of the tapping tests, all the indications 
 of power of control here enumerated show superiority in girls. 
 
 Summary. — On the basis of the data here presented it 
 seems fair "to conclude that the physical development of girls 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 313 
 
 is relatively more advanced than that of boys at the age of 
 five or six or thereabouts. On the average they have then 
 attained a greater percentage of their post-pubescent height, 
 a greater percentage of their post-pubescent weight, their 
 skeletal development is more advanced, and their dentition 
 has progressed further. Aside from this there is a strong 
 suggestion that a greater percentage of adult brain weight 
 has been attained by them, and this is supported by the fact 
 that most data available show a more advanced power of 
 neuromuscular control. How great the difference in general 
 physical development is a matter to be determined by fur- 
 ther investigation. 
 
 Discussion, Pedagogical Suggestions, and Summary 
 
 So far, the aim has been to find solutions for our two main 
 problems. For the first we may say that, while not proved, 
 a great mass of data indicate a transition in physical develop- 
 ment at the period of usual school entrance. Regarding the 
 second we may say that greater advancement in the physical 
 development of girls at this time is pretty clearly shown. 
 
 The solution or partial solution of these two problems sug- 
 gests a number of others. If this is a transitional period, 
 what is its fundamental nature? Is the transition general or 
 due to a single factor, as dentition? That it is the former 
 seems to be the case, but it is still an open problem. In case 
 the transition is not of a general, fundamental nature, then 
 a whole series of problems regarding the relationship of these 
 several transitional phenomena arise. For example, is ra- 
 pidity of dentition to be correlated with rapidity of skeletal 
 ossification, as Holt (45) has stated is the case in infancy? 
 Are disturbances of growth in height and weight noted at 
 this period due to conditions of dentition or do they occur 
 regardless of it? 
 
 The correlation of mental with physical development at 
 this period is a problem which those working with mental 
 tests cannot afford to neglect. Granted a degree of physical 
 and mental correlation, and that this period is a transitional 
 one for physical development, then the whole question of 
 qualification for school entrance is thrown open anew. The 
 problem is still further complicated by the fact of great 
 variability shown in the developmental factors at this time. 
 
 In view of their close relationship to other phases of physi- 
 cal development, and in the absence of definite methods of 
 grading, the question arises as to whether the time of ap- 
 pearance of the first molars could not be cited as a point 
 before which school duties should not be imposed. 
 
314 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 The differences of the sexes at this period also suggest 
 some important problems. Recalling that the female reaches 
 maturity some three years earlier than the male — or nearly so 
 — and that she reaches the pubescent stage almost two years 
 earlier, on the average, the fact that she has progressed fur- 
 ther at the school entrance age is to be expected. The ques- 
 tion arises here, however, as to how early the differentiation 
 of the sexes actually begins. Since differences in the averages 
 of absolute measurements are noticeable at birth, and in view 
 of the popular opinion that the prenatal period is shorter for 
 females than for males, one is tempted to raise the question 
 as to whether differentiation does not actually begin before 
 birth. 
 
 The problem of the amount of difference between the sexes 
 comes up, and also the further one of working out a method 
 sufficiently refined to measure them. The question as to 
 what this difference signifies for the pedagogy of this period 
 of life is an extremely important one. 
 
 The preceding facts and conclusions form the basis for 
 some further conclusions and suggestions for the hygiene and 
 pedagogy of the early years of childhood. 
 
 We have shown that boys and girls of equal age are not 
 equally advanced physically at the school entrance period of 
 life. We have presented a mass of evidence in support of 
 the conclusion that this is a transitional period in the physical 
 development, referring to the great individual variability, as 
 indicated by dentition, skeletal development, etc. Associat- 
 ing these facts with the indications of a correlation between 
 mental and physical development, the need of a physiologi- 
 cal grading for entrance to school becomes strikingly evident. 
 Granted a proper grading for entrance on this basis, the grief 
 and the expense of " repeaters " up through the grades would 
 be tremendously lessened. School entrance should be on the 
 basis of the fitness of the individual, not on the basis merely 
 of his chronological age. 
 
 Again, the rapid growth, the ease with which the delicate 
 balance of the organism may be overthrown, the lasting detri- 
 mental results following the wrong " inclination of the twig," 
 all suggest that the thing of supreme importance in these 
 early years is rather health and development than formal 
 school work. The paramount importance of habits of health is 
 shown over and over again in case of the teeth. Lack of the 
 habit of cleanliness of the teeth is followed by caries, loss 
 of bicuspids or all important first molars, with probably ill- 
 fitting permanent dentures, deformed facial features and in- 
 jury to health. Habits of thumb-sucking, mouth-breathing, 
 
PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 315 
 
 and the like, may cause malocclusion and abnormal growth 
 of jaws and related facial parts. Good permanent tooth 
 crowns are largely dependent upon fresh air, exercise and 
 good nourishment throughout the first five or six years of 
 life. Attention to all these things should be first until after 
 the organization of the childish body is more nearly complete. 
 
 The eyes are unfit for close visual work until after the 
 transitional period. 
 
 Adenoids left uncared for in these early years are liable 
 to cause disturbances of development. According to Yearsley 
 (104) the time to have them removed is after six months 
 and before six years of age. 
 
 Enlargements of the tonsils, with decreased power to resist 
 invasions of micro-organisms, accompanies eruption of the 
 first molars, in many cases (100, p. 13). Until after this 
 time, no child should be confined to the school room where 
 his chances of exposure to infectious disease germs are in- 
 creased. 
 
 In view of all these things, this survey would be worth 
 while, even though it did no other thing, could it portray in 
 its true light the importance of all the years of earlier child- 
 hood for the normal, healthy physical development of the 
 child. 
 
 From a different viewpoint another suggestion is in place. 
 Could " types " be agreed upon for use in comparisons of 
 stature, and like physical characteristics, similar to these used 
 in anthropometry, much more accurate knowledge of develop- 
 ment might be gained. Grouping of material according to 
 types, combined with comparisons of relative advancement, 
 such as we have used above, should lead to more satisfactory 
 results in indicating stages of development. 
 
 Final Summary. 
 
 The main conclusions to which the facts presented in this 
 thesis point are : 
 
 1. That there is a mass of evidence indicating a transition 
 in the physical development of the child at the period of usual 
 school entrance. 
 
 2. That there is evidence to show that girls are on the 
 average more advanced in their physical development at this 
 period. 
 
 5. That requirements for entrance to school based on stage 
 of progress in physiological development, or physiological 
 age, would be far superior to a requirement taking account of 
 chronological age only. 
 
3l6 PHYSIOLOGICAL AGE AND SCHOOL ENTRANCE 
 
 4. That in the appHcation of physical measurements for 
 the purpose of finding degree of progress in physical develop- 
 ment, a method indicating the percentage of adult size at- 
 tained at a given age is superior to a method comparing 
 absolute data directly. 
 
 5. That the hygiene and pedagogy of this period are of 
 prime importance for the future development and welfare 
 of the individual. 
 
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