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I »

• '

THE MUSCULUS STERNALIS AND ITS OCCURRENCE
IN (HUMAN) ANENCEPHALOUS MONSTERS. By
Francis J. Shepherd, M.D., CM., Professor of Anatomy
in M'Gill University, Montreal. (Plate XV.)

At the meeting of the British Association held %8t summer in
Montreal, Professor D. J. Ciinningham, of Dublin, read a paper
on " The Value of Nerve-Suppl" in the Determination of Mus-
cular Anomalies," in which he stated his belief that the muscu-
Ins sternalis belonged to the pectoral group — in fact, was an
aberrant portion of the pectoralis major, as recently suggested
by Mr Abraham, of Dublin.^ Professor Cunningham had traced
the nerve-supply ^ of the museulus sternalis in five cases, and
found that it came from the internal anterior thoracic nerve,
a proof he thought that it belonged to the pectoral group. He
also threw out the suggestion that this might possibly be a new
inspiratory muscle antagonistic to the triangularis sterni appear-
ing in man (for it acted when well developed as an elevator of
the ribs), and stated his impression to be that it occurred more
frequently in females, as costal inspiration is more pronounced
in women than in men.

In the discussion which followed, both Dr G. E. Dobson and
myself held that the musculus sternalis was most likely a
remnant of the panniculus caruosus. Dr Dobson considered
that the sterno-cuticularis muscle of the hedgehog closely corre-
sponded to the musculus sternalis.

Professor Cunningham also mentioned in his paper that Mr
Abraham had recently found the musculus sternalis to occur
very commonly in anencephalous monsters, as he had seen it in
six out of eleven specimens examined.

Since the meeting of the British Association I have examined
six anencephalous monsters which are in the museum of the
Medical School of M'Gill University, and have found in each
one a well-marked example of the musculus sternalis. My
recent dissections of these monsters has had the effect of changing

VOL. XL\.

^ Trans. Acad. Medicine in Ireland, vol. i., 1883.
' Jour, Anat. and Phys., January 1884.

312

DR FRANCIS J. SHEPHERD.

my previous views in regard to the homology of this musole. T
have been conviuced that it does not belong to the panuicuUis
group, but very probably should be classed with the pectoral
group for the following reasons : —

1. In seven out of the nine muscles found in these monsters
(three had double muscles) the nerve-supply was furnished by
the anterior thoracic; one of these seven, however, in addition,
received a small branch from one of the intercostal. In the
other two muscles, occurring in the same foetus, I was unable to
satisfactorily make out the nerve-supply, but am inclined to
believe it came from the anterior thoracic (Case III.).

2. In three the fibres of the abnormal muscles were continuous
with those of the greater pectoral (figs. 1, 2, 6), and in one
(fig. 5) the fibres pierced tbe greater pectoral.

3. In several the insertion of the musculus sternalis was
covered by the pectoralis major, and the origin was in common
with the upper sternal fibres of the pectoralis major (figs. 1,

4, 6).

4. The greater pectoral was deficient on the side on which the
musculus sternalis was present in eight cases (figs. 1, 2, 3, 5, 6).

5. In one (Case VI.) the right platysma myoides was well
developed, and passed some distance below the clavicle. It was
separated from the musculus sternalis of that side by fascia and
a thick layer of fat, and was on a plane quite superficial to the
musculus sternalis.

In all the cases except one (fig. 3) the abnormal muscle was
quite large and well developed, and had an attachment to the
sternum and costal cartilages. The majority of the muscles
were triangular in shape, though some were fusiform. In the
last three dissected I had no difficulty in tracing the nerve-
supply, as the nerve was always found passing along the interval
which existed between the two portions of the greater pectoral,
thence over the pectoralis minor, through the costo-coracoid
membrane, to the internal anterior thoracic nerve. The nerve
always entered the muscle on its deep surface. In three of the
subjects the muscles were continuous with the sternal insertion
of the sterno-mastoid (figs. 1, 2, 4). In two a portion of the
mu^icle blended with the aponeurosis of the external abdominal
oblique.

*»' :

MUSCULUS STEHNALIS IN ANKiNCEPUALOUS MONSTERS. 313

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1 do not propose in this paper to discuss all the various views
that have been held in regard to the homology of the musculus
sternalis, as this has already been ably done by Professor Turner^
and others; but I might mention that Professor Bardeleben^
has advanced the theory that some of these muscles belong to
the sterno-mastoid, and are supplied by the intercostal nerves,
whilst others should be classed with the pectoral group, because
they receive their nerve-supply from the anterior thoracic.
Malbrane's^ observations agree v.'ith Bardeleben's, for in two
living subjects he found the musculus sternalis standing out
quite perceptibly under the skin.

In the first case faradisation of the intercostal nerves brought
the muscle into action, but in the second it failed ; but when
faradisation of the thoracic nerves was employed, the muscle
responded immediately.^

M. Teatut* holds that the musculus sternalis (pre-sternal) is
in its upper part an appanage of the sterno-mastoid, and in its
lower belongs to the external abdominal oblique. He says these
muscles (sterno-mastoid and external abdominal oblique) are in
the same mu'^cular plane, and that the musculus sternalis is the
remnant in man of the old connection which formerly existed
between the two — a connection which exists in serpents.

As I said above, I feel disposed to consider the nmsculus
sternalis as belonging to the pectoral group, and await further
light to determine its proper morphological significance. In
some of my cases lb appeared to take the place of the absent
portion of the greater pectoral, and where the muscle was well
developed would act as an elevator of the ribs.

As to its occurrence in anencephalous monsters, I am unable
to afford any explanation. As far as I can judge from the six
specimens I have examined, it appears to be the normal condi-
tion. There seems to be great variety of origin, insertion, size,
and shape of these muscles, no two being exactly alike. The
fact that this muscla occurs so commonly in the brainless mon-
sters would point rather to its being a rudiment than a new

* Jour, of Aiiat. and Phys. , vol. i, p. 246.

^ Quoted by Testut in Les Anomalies Miisculaires chez Vhomme, 1884.
' In Case III. in my series the muscle was supplied by both intercostal and
anterior thoracic nerves.

* Les Anomalies Mioscidaires chez I'hovime, p, 84.

;n4

DR FRANCIS J. SHEPliraU*.

muscle appearing in man. For it is in these cases of arrest of
development we should expect to find reversions, rudiments, and
nomalies. On the other hand, no arrangement of any existing
pectoral group resembles that found in these brainless mon-
sters.

The proportion of female anencephalous monsters is very large
in my series — five out of six are females ; and as far as I can
learn it is rather the exception for an anencephalous monster to
be of the male sex.

The cases described in detail are as follows : —

' k

Case I. (fig. 1). Female fcetus, full term. Anencephalous.
Musculus sternalis unilateral. Left.

The musculus sternalis in this specimen is of large size, and
arises from the fascia over the first piece of the sternum by a flat
tendon, which is continuous above with the sternal origins of
both sterno-mastoid muscles, and on the right sid'3 is connected
with the muscular fibres of the greater pectoral crising from the
manubrium. From this origin the muscle passes downwards
and outwards to the left side, expanding as it desc^nds into a
large fusiform muscle, which is inserted into the whole of the
fourth left costal cartilage and into the side of tlie sternum
opposite the fifth and sixth cartilages ; the innermost portion of
the muscle is prolonged downwards over the lower part of the
greater pectoral, and ends in the aponeurosis of the external
oblique muscle of the abdomen.

T!ie abnormal muscle lies on the sternum and costal cartilages,
and has only a few of the deeper fibres of the greater pectoral
beneath it. Above, on the outer edge, some muscular fibres
came off from the musculus sternalis, and passing outwards
form part of the greater pectoral muscle.

The nerve supplying the muscle enters its under surface about
half-way down the muscle ; it can be traced outwards through
a cellular interval in the greater pectoral, over the pectoralis
minor, to its upper border, where it pierces the costo-coracoid
membrane, and joins the internal anterior thoracic nerve. As it
lies between the two pectorals it gives off a branch to the lower
part of the great pectoral.

MUSCULUS STEKNALIS IN ANENCEPHALOUS MONSTERS. 315

Case II. (fig. 2), Female fcehis. Anencephalous, loith spina
bifida of cervical and upper dorsal ret/ions. Musculus sternalis
hilateral.

The two muscles have a common origin from the first piece of
the sternum, wliich is continuous above with the sternal portions
of both sterno-mastoid muscles. The left muscle, smaller than
the right, consists of a fiat narrow band of muscular fibres, which
pass down from the common origin to be insei^ed into the third
left costal cartilage and side of the sternum. At its insertion it
is covered by the fibres of the lower segment of the greater
pectoral. The ri(/ht muscle is large and flat, and, besides the
origin common to it and its fellow, is attached to the sternum
opposite the second and third costal cartilages. It divides into
three sets of muscular fibres — the outer inserted into the upper
border of the lower segment of the peotoralis major, the middle
continuous with the fibres of that muscle, and the inner inserted
into the lower end of the sternum and upper part of the ensi-
form cartilage. On both sides a triangular portion of the greater
pectoral is absent ; the spaces thus left are partly covered by the
abnormal muscles. This space is longer on the left than the
right side. In this fcetus, owing to its very friable condition, I
was unable satisfactorily to trace the nerve-supply of these
inomalous muscles, but am inclined to believe that the nerve-
supply comes from the anterior thoracic, as on each side I traced
a branch from the anterior thoracic over the lesser pectoral to
the triangular interval between the two segments of the great
pectoral, but there I lust it.

Case III. (fig. 3). — Female fcetus. Anencephalous ivith spina
bifida of cervical region. Musculus sternalis unilateral. Left side.

In this case the abnormal muscle consists of a small fusiform
slip which arises from the sternum opposite the second costal
cartilage by a thin aponeurosis, passes down over the left greater
pectoral a little outside the sternum, and finally expands into a
broad aponeurosis, which blends with the fascia over the external
abdominal oblique. It receives its nerve supply from two
sources. The larger nerve, which enters the middle of the
muscle, can be traced through the greater pectoral over the
lesser pectoral, and through the costo-coracoid membrane to the

816

DR FHANCIR .1. SnEPHEKD.

internal anterior thoracic. The smaller enters the muscle nearer
its upper end, and can be traced through the intercostal space to
the third intercostal nerve. Both nerves supply the muscle from
its deep surface. This is tlie only case where a branch from the
intercostal could be traced to the muscle itself. In several of
the other cases the intercostal nerves pierced the muscle, but
gave no branches to it. No portion of the greater pectoral is
absent in this case.

X /

Case IV. (fig. 4). — 3Iale fcetiis. Anencephcdous ivith spina
hifida of cervical region. Musculus sternalis unilateral. Left.

The abnormal muscle in this fcetus is of large size, flat and
triangular, arises by a tendon from the manubrium, in common
with the upper sternal portion of the right pecturalis major and
the sternal portion of the left sterno-mastoid with which its left
border is continuous. As it passes down to the left it soon ex-
pands into a broad muscle which is inserted into the third
costal cartilage. At its insertion it is covered by the fibres of
the pectoralis major. Its inner edge is prolonged downwards
over the lower portion of the last mentioned muscle. On the
left side a triangular portion of the pectoralis major muscle,
arising from the upper part of the sternum and costal cartilages
of the second and third ribs, is wanting, the space left being
partly covered by the musculus sternalis.

The nerve supplying the muscle can be seen crossing the
triangular interval, and can be traced, as in the other cases, to
the anterior thoracic.

Case V. (fig. 5). — Female fcetus. Anencephalous in sjmia
bifida. Musculus sternalis bilateral.

Both muscles arise in common with the upper sternal fibres of
the pectoralis major from the manubrium, and diverge from each
other as they descend.

The left muscle passes down over the sternum and left costal
cartilage, and is inserted into the fourth .ostal cartilage near the
sternum. It is a flat triangular muscle of considerable size.
Continuous witli its lower fibres, and rr-^ing along its inner
edge, is a small muscular slip which has an attachment above by
a round tendon to the middle of the sternum, passes over tlip

MUSCULUS STKUNALIS IN AXENCKPHALOUS MONSTERS. 317

lower part of the greater pectoral, and is inserted into the fascia
covering that muscle. A large portion of the central part of the
pectoralis major is absent, the space left, as in the other cases,
being partly covered by the abnormal muscle. The nerve supply-
ing the muscle crosses this vacant interval, and can be tranced, as
in the other cases, to the internal anterior thoracic.

The rijht muscle goes down and out from the common origin,
and soon divides into two slips, the outer of which, after piercing
some fibres of the greater pectoral, is lost in the fascia covering
that nmscle. The iinier slip continues down immediately to the
right of the sternum, and ends in a tendinous expansion which
is inserted into the fascia of tiie lower part of the pectoral
muscle. On this side also the portion of the great pectoral is
deficient which arises from the second and third costal cartilages
and the corresponding portion of tlie sternum. The nerve can
be traced crossing the triangular interval, and under the upper
segment of the greater pectoral to join the anterior thoracic above
the lesser pectoral.

Case VI. (fig. 6). — Female fcetns. Anenceph'.lous with spina
hijida of cervical and upper dorsal regions. Musculus sternalis
hilatercd.

The muscles of the two sides have a common origin from the
manubrium.

The right muscle, triangular in shape, is the larger. It soon
becomes muscular, crosses the triangular interval caused by
absence of a portion of the great pectoral, and is inserted by
muscular fibres into the upper border of the lower segment of
the greater pectoral, and also into the slernum oppjsite tlu;
fourth costal cartilage. Some of its fibres pass over the pectoral
muscle and blend with it. As in the other cases, it is supplied
by a branch from the internal anterior thoracic nerve, which
reaches the muscle in the usual way.

The left muscle divides into two portions, the outer of which
is the larger, fiat and ribbon-shaped, passes down over the
triangular interval between the upper and lower segment of the
greater pectoral, and is inserted into the third costal cartilage ;
the inner portion has an additional origin from the second piece
of the sternum. It continues down, over, and to the left side of

318

DR FRANCIS J. SHEPHERD.

the sternum, developing into a fusiform-shaped muscle, which
ends by dividing into two tendinous slips, one of which is
inserted into the lower end of the sternum, and the other into
the fascia covering the pectoralis major. The nerve-supply is, as
in the other cases, furnished by a branch from the internal
anterior thoracic nerve, which joins the deep surface of the
muscle after pursuing the usual course across the lesser pectoral
and vacant interval between the two parts of the greater
p 'oral. In its course a small branch is given off, which goes
to ihe lower part of the greater pectoral.

On each side there is a deficiency of the great pectoral, a
triangular portion arising from the second and third costal
cartilages being absent. The interval is larger on the right
than the left side, and on each side is partially covered by the
abnormal muscle.

In this foetus on the right side the platysma myoides is
strongly developed, continues over the clavicle, and reaches for
some distance below it. It is a well-developed muscle, and is
separated from the musculus sternalis of that side by fascia and
a thick layer of adipose tissue, so that it is on a plane quite
superficial to the musculus sternalis.

Note. — I have, in adults, only seen the musculus sternalis three
times ^ in three hundred subjects. Some cases, no doubt, escaped
my notice, owing to the majority of the subjects having been
injected through the heart, and, in consequence, the sternum
having been sawn through the centre. In all the cases seen the
muscle was well developed. In one CL-a it was continuous
above with the opposite sterno-mastoid, and below was attached
to the cartilage of the fifth rib. In the second case it arose
from the second costal cartilage, and^ passed down over the
pectoral muscle, and ended by being inserted into the fa&t.a
covering that muscle. Some of its upper fibres intermingled
with those of the platysma myoides. The subject was very thin.

In the third case the muscle was attached above and below to
the fascia covering the greater pectoral. All three muscles
occurred in males. Two of the muscles were on the left side
and one on the right.

^ Annnh of Anatomy and Surgery, 1881-83.

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MUSGULUS STERNALIS IN ANENCEPHALUUS MONSTERS. 319

EXPLANATION OF PLATE XV.

Fig. 1. Left musculus sternalis, arising from the first piece of
sternum, and continuous with the sterno-mastoids, inserted into the
'\ i fourth costal cartilage and side of sternum — a portion continuous

with the left pectoral. Nerve-supply from anterior thoracic. (Female.)

Fig. 2. Douhle sternalis muscle, arising from manubrium, and, in
common with upper fibres of great pectoral and sterno-mastoid, inserted
on right side into sternum and great pectoral, on left into third costal
cartilage. Sternal and costal origins of both greater pectorals defec-
tive. (Female.)

Fig. 3. A slender left musculus sternalis, arising from sternum
opposite second costal cartilage, and inserted into the aponeurosis of
external abdominal oblique. Supplied by a branch from anterior
thoracic nerve and intercostal. (Female.)

Fig. 4. Left musculus sternalis, arising from manubrium, in
common with sterno-mastoid and upper fibres of greater pectoral
inserted into third costal cartilage. Nerve-supply from anterior
thoracic. Left pectoralis major deficient in central part. (Male.)

Fig. 5. Double musculus sternalis, arising from manubrium, with
upper fibres of greater pectoral on right side. Two slips piercing
pectoral muscle, and inserted into aponeurosis covering that muscle.
Muscle on left side inserted into fourth costal cartilage. Nerve-supply
from anterior thoracic on both sides. (Female.)

Fig. 6. Double musculus sternalis, arising from manubrium. Eight
side flat muscle inserted into greater pectoral and sternum. Left side,
two slips — one inserted into third costal cartilage, and other into
aponeurosis of greater pectoral. Nerve-supply from anterior thoracic
on both sides. Both greater pectorals defective. (Female.)