A TREATISE OF THE REASON OF Muscular Motion: Or the Efficient Causes of the Contraction of a Muscle.  
WHEREIN Most of the Phaenomena about Muscular Motion are explained.  
By RICHARD BOULTON, of the City of Chester, Medicine. Proficiens.  
〈◊〉 〈◊〉 〈◊〉 〈◊〉 〈◊〉.  
LONDON, Printed by A. and J. Churchill, at the Black Swan in Pater-Noster-Row, 1697.   

TRactatum hunc cui Titulus (A Treatise of the Reason of Muscular Motion, etc.) dignum Censemus, quî Imprimatur.  

Samuel Collins, Praeses.  
Thomas Burwell, Sen.  
Richard Torless,  
William Dawes,  
Thomas Gill, 

Censor.     
Datum in Comitiis Censoriis ex Aedibus Collegii nostri. Sept. 11. 1696.   

TO THE MOST LEARNED AND EVER HONOURED Dr. Collins, Precedent: 

Dr. Thomas Burwell,  
Dr. Richard Torlesse,  
Dr. William Dawes,  
Dr. Thomas Gill.    
CENSORS OF THE College of Physicians; This Treatise is Humbly Dedicated  
By Your Most Faithful And Obedient Servant R. BOULTON.   

TO THE READER.  
I Have examined this Treatise with the strictest Reason that I am Master of; and it appears to me Reasonable, in every Particular.  
But I am altogether desirous of Truth, and would not out of any fond Opinion of my own Hypothesis, receive it sooner than another Man's. I am most inclined to suspect whatever is a Faetus of my own Brain.  
If it may be any ways serviceable to others, I have my Desire.   

THE CONTENTS.  

THE Usefulness of Muscular Motion as it tends to the Maintenance of Life. Page, 1  
— With respect to the Soul and the Animal Functions. p. 3  
Dr. Willis's Opinion examined. p. 7  
Mr. Cowper's Opinion examined. p. 20  
Dr. Ridley's Opinion examined. p. 30  
The Structure of a Muscle. p. 40  
The Formal Cause of the Contraction of a Muscle. p. 41  
Vessels implanted into a Muscle. p. 42  
Their Distributions. p. 43  
And Terminations in Small Glandules. p. 45  
What a Gland is. p. 47  
The Use of the Nerves. p. 52  
— And Arteries inserted in the Glands. p. 53  
— To prepare a Subtle Liquor. ibid.  
The Use of it. p. 54  
The Reason of Tonick Motion. p. 59  
— Of Local Motion. p. 60  
The Use of the Lymphducts, p. 68  
The Nature of the Liquor, and the Manner of its Preparation. p. 69  
— By Fermentation. p. 70  
— Proved. p. 72  
The Reason of Involuntary Motion. p. 82  
Why the Systole of the Heart is interrupted with frequent Diastoles. p. 86  
Why the Muscles serving to Respiration are in some Measure Subject to our Appetite. p. 90  
Some Phaenomena about Muscular Motion Explained. p. 93  
The Reason of Convulsions. p. 103  
— of the Palpitation of the Heart. p. 109  
— of the Cramp. p. 111  
— Of the Subsultus Tendinum in Fevers. p. 113  
— Of Spasmus Lynicus. p. 114    

OF THE REASON OF Muscular Motion: OR, The Efficient Causes of the Contraction of a Muscle, etc.  
IF we consider the Usefulness of Muscular Motion, 
The Usefulness of Muscular Motion, as it tends to the maintenance of life.  with respect to an Individual; and that, either as it tendeth to the maintenance of Life, or the perfection of Man, that is, with respect to the Soul, and the Animal Functions; it is the very spring upon which all our Actions, both Natural, Vital and Animal wholly depend. By the Assistance of Muscular Motion, all the parts of our Body perform their particular Offices: Our Food is prepared by Mastication, and conveyed to the Stomach; the concocted Chymus is thence expelled into the Intestines, where the purest Chyle is separated from the impurer Faeces; every peculiar Liquor circulates through its distinct and proper Vessels The Chyle through its Lacteals and the Lympha through its Ducts; and the Mass of Blood is disributed through the Arteries, that all the Parts of the Body, both Principal and Subsubservient, Noble and Ignoble, may receive their proper Liquors: In a word, it is by this very Action of Muscular Motion, that 〈◊〉 〈◊〉 〈◊〉 〈◊〉 〈◊〉.  
If we respect the Animal Functions, 
With respect to the Soul and its Functions.  they so much depend upon Muscular Motion, that when the Heart ceaseth to move, all the Faculties of the Soul are presently extinguished. The Faculties of the Soul are not only dependent on Involuntary, but also Voluntary Motion; without which, both our Eyes and Ears would be unfit to receive external Objects; and Man would be endowed with a Sensitive and Rational Soul to no purpose. Our outward Senses would in vain receive external Objects, unless we had an Intelligent Faculty; in vain should we Understand, unless we had Reason to distinguish good from evil, that we might desire the one, and contemn the other: In vain also would be the Dictates of our Reason, if we had not a Loco-Motive Faculty, to pursue those things we desire, and to flee from those things which we are willing to avoid. But by Muscular Motion, the Dictates of our Reason, accordingly as they are biased by our Appetites, are put into Action, and the intimate results of all our Faculties are brought to light. Muscular Motion is the very Index Animi, by which the Temper and Disposition of the Animal Faculties are made evident; it's the Ultimate Result of our most secret Thoughts and of our Will; so that if we weigh but the mutual dependence of all the Parts of a Man's Body, 
A Muscle is a Noble part.  there is no Reason, that a Muscle should be termed an Ignoble part; since by the loss of a single one, either the Natural, Vital, or Animal Faculties are in some measure vitiated.  
The Stomach, the Liver, the Spleen, the Lungs, etc. are all subordinate and subservient to the Principle Faculties of the Soul; in as much as they by their mutual Symbolums, prepare the Pabulum of the Animal Spirits. But the Muscles designed for Motion, especially Voluntary (in as much as the most perfect and refined Actions of the Soul, are performed and expressed by Muscular Motion,) are of a more noble use, the perfection of Man consisting more immediately on the energy and execution of the Principal Faculties of the Soul, than in the subordinate and subservient Faculties, both Natural and Vital.  
For as much as the Life and Perfection of Man, so much depend on Muscular Motion, I shall inquire into the Efficient Causes of it: But before I propose my own Hypothesis, it is necessary that I first confute what has hitherto been said on this subject; nevertheless, because it would be too tedious to examine the Opinions of so many Authors, I will only show the Errors of Two or Three of the latest and most considerable.  
The most Learned and Famous Dr. Willis Conjectures, 
Dr. Willis's Opinion examined.  that the Spirits are conveyed to the Tendons of a Muscle, and are there plentifully laid up as in a convenient Storehouse; which Spirits being of an Agile and Elastic Nature, as far as they are able, and are permitted, always endeavour to expand themselves; and that when an Instinct to perform Muscular Motion, is carried from the Brain or Cerebell, to this common Storehouse, they presently leap out of the Tendinous, into the fleshy Fibers, where they meet with active Principles of another Nature, supplied by the Mass of Blood; which two strongly & mutually fermenting, stuff up and tumify the Fleshy Fibers; whence proceeds the Contraction of a Muscle. When the Contraction ceaseth, he thinks that most of the purest Spirits that remain, return into the Tendons, the other more impure Particles remaining amongst the Fleshy Fibers; he fancies, that the Fibers of the outward Membrane contracting, promote the Retrocession of the Spirits into the Tendons; and that for a continual supply of these Liquors, the one is always stored up in the Tendons, through the Nerves; and the other in the Carnous Fibers, from the Mass of Blood.  
This Conjecture as it seems ingenious, 
No Elastic Spirits laid up in the Tendons.  and at the first view very mysterious, yet upon a more strict Examination, it appears altogether as improbable. For to suppose that these Elastic Spirits are laid up, and stored in the Tendinous Fibers, is contradictory to Reason, as well as the Laws of Circulation; for since in every Systole of the Brain, a fresh supply of Animal Spirits is sent into all the Branches of the Nerves, those Animal Spirits in the Tendinous Fibers, must needs by a succession of new matter, be pressed forwards into the Fleshy Fibres; and consequently Accidental Contractions must follow: for as much as there are no Valves to hinder their immediate Passage. And that there are no Valves, nor any thing else to stop the continual Passage of these Spirits, may be argued hence; because whatever would stop the protrusion of these Spirits, when pressed by the Systole of the Brain, would also hinder 'em from rushing out of their Storehouse, though stirred up by an Instinct, and so the Muscle would be deprived of Motion.  
But supposing that it were possible for these Elastic Spirits, to be stored up in the Tendinous Fibers, and that neither their agility, and continual Endeavours to explosion, nor the force of succeeding Spirits would drive 'em out; yet when a Muscle ceaseth from Motion, perhaps a Day or Two; the Tendon would be so filled and stuffed up, that being no longer capable of receiving more Spirits, either they must be forced forth into the Carnous Fibers, and cause violent and Involuntary Contractions; or regurgitating and flowing back upon the Brain, would there cause a Vertigo, or some other more fatal Disease; and besides the Spirits by stagnating so long, would be chilled and coagulated, and thereby rendered unapt for Motion.  
It is not only absurd to imagine, 
Nor instinct sent from the Brain, because  that the Tendons, are Storehouses for Spirits; but also that they, the Instinct being given, should cush out of the Carnous Fibers; one might as well think, that the Contents of the Stomach, the Gall in the Vesica Bilaria, or the Urine in the Bladder, might be excluded by instinct, merely without the help of Muscular Coats; or that the Circulation of the Blood might be carried on, without the Systole and Diastole of the Heart.  
Moreover, 
Either it implies, that the Spirits are Intelligent,  to allow, that those Elastic Spirits do after this manner rush into the Carnous Fibers, when the Instinct is given from the Brain, or to speak more properly, ad imperium animae, either implies, that those Spirits, which Dr. Willis calls Materiam subtilem, are intelligent Matter; that they are able to understand the Dictates and Commands of the Soul; and to put them in Execution, proprio motu; or that some Spirits of a Different Nature, 
Or that Spirits of a Different Nature are sent from it.  are at the command of the Soul, sent from the Brain, which cause the Spirits in the Tendons to be uneasy, and by exagitation to expand themselves more powerfully, and consequently to leap forth. But if neither the texture of those Spirits, which as Dr. Willis says, give the Instinct, be changed, and different from those in the Tendons, nor the Subtle Matter be Intelligent, there would be no Explosion, or leaping forth of the Spirits out of the Tendons, either by instinct, or the accession of new Matter from the Nerves; except, as I noted before, the Spirits which filled the Tendinous Fibers, were driven, and successively forced out by a fresh supply of Spirits, sent in by every Systole of the Brain; and so, Muscular Motion would not be Voluntary, but Accidental.  
That this Matter is not Intelligent, is proved sufficiently; 
The Matter is not Intelligent.  because to allow that this acts as it receives Instinct from the Brain, is to allow a subordinate Soul which acts proprià facultate in every Tendon; which is false; for an Intelligent Faculty in a Tendon, which must act according to the Dictates of the Principal Soul, cannot be supposed to be a Faculty of that Soul, no more than the Soul of a Disciple, Juratus in Verba Magistri, and who exactly follows the Dictates of his Master, can be said to be a part or faculty of his Master's Soul.  
That no Spirits of a different Nature from those he supposes to be in the Tendons, 
No Spirits of a different Nature are sent from the Brain.  can be transmitted from the Brain, is evident. For since the same Spirits are distributed through all the Nevervous Channels, from the same Fountains, the Brain, or the Spinal Marrow, whatever causes the Spirits to leap out of one Tendon, would cause the same Effects in all, and all the Muscles must be contracted at the same Instinct.  
Having thus confuted the Grounds and Foundation of his Hypothesis, it's not irrational to expect, that the whole Superstructure should fall: I shall only further take notice,  
First, 
No active Principles heaped up in the Fleshy Fibers.  That if Active Principles of another Nature were heaped up in the Fleshy Fibers, by degrees they would be so stuffed and swelled up, that there would be no room for the entrance of Elastic Spirits from the Tendons; but they themselves would rather, by reason of their quantity, be forced into the Tendinous Fibers.  
Secondly, That it would be absurd to imagine, 
Nor retrocession of subtle Matter into the Tendons.  how these Two Spirits after so intimate a commixture, as would be produced by a strong Fermentation, can be so easily separated, that the one should suddenly run back into the Tendons, leaving the other behind in the Fleshy Fibers: For when Two Liquors of a different Nature, and whose Minute and Subtle Particles have a different Motion, are mixed together; by a mutual Fermentation is produced so intimate a mixture, that it is impossible to separate the one from the other sincere; because by a mutual collision of different Particles, both must needs in some measure be altered, and degenerate from their Pristine State, so that if the purest part of these Spirits remaining should run back into the Tendons, they would be disagreeable to, and different from those purer Spirits sent from the Brain; and would infect the Storehouses with such a fermentative taint, that as new supplies were laid down by the Nerves, fresh Explosions and Contractions of the Muscles must follow.  
Thirdly, To help the Retrocession of these Spirits into the Tendinous Fibers; Dr. Willis would have the Fibers of the investing Membrance to contract: But I should rather think, that they would contract at the same time with the Muscle, if the Instinct were carried through the same Nervous Channels with the Spirits; because, as he says, they are first laid down in the Membrane, and thence conveyed to the Tendon?  
Mr. Cowper, in the Introduction to his Myotomia Reformata, 
Mr. Cowper's opinion examined.  suspects; that the Blood is a Pondus, by which the action of a Muscle is performed: The grounds of this suspicion are chiefly taken from two Experiments: The first is a Ligature on the descending ttunk of the Aorta, whereby all the inferior Parts, became destitute of Motion, which they recovered again when that Ligature was loosed: The second, is to be taken for a more surprising Argument, because after a cessation of Motion, the Muscles of the Legs renewed their Contractions, by an Injection of Water into the Crural Artery.  
But that there is no Reason to assert, that the Blood acts as a Pondus from these Experiments, will hereafter sufficiently appear, by giving other Reasons for these Phenomena.  
In the mean time to evince, 
The Blood does not act as a Pondus.  that the Blood as a Pondus does in no wise help the Contraction of a Muscle, but does on the contrary, much interrupt and resist the Intumescence of the Musculous Fibers, 
The First Argument.  by which the Contraction of a Muscle is performed, let any one hold his hands before a Fire, with his Arms perpendicular, continuing 'em in that posture till he feels the Musculous Parts well filled with Blood; then lifting up both his Hands, let him hold 'em up as high as he can, till the Pondus of the Blood be drained, and the Veins which before were full and tumid become flaccid and empty, and he'll move his Fingers much more easily and swifter, when the Muscles are eased of the Pondus of Blood, than he did whilst his Hands were dependent; yet certainly there was a greater Pondus of Blood upon the Muscles in the former posture, whilst they were tumid, than in the latter when emptied of superfluous blood.  
Another Argument to prove, 
A Second.  That the Pondus of the Blood doth not at all promote the Contraction of a Muscle; may be taken from the Observation of Cacochymick Bodies, who are far more dull and unapt for Motion than healthful people, whose Veins and Arteries are moderately filled with Blood, which is not so thick and heavy; for in Cacochymick Bodies, there is a greater Pondus of Blood, but because it doth not yield matter for to swell the Carnous Fibers, as plentifully as blood of a more healthful constitution, Motion is not so brisk and strong; whereas if the Pondus of the Blood did any ways contribute to the performance of Muscular Motion, they would be far more strong; for the thicker the Blood is, the greater is the Pondus.  
In the Muscles of the Legs and Thighs there is always, 
A Third.  when the Body is erect, a greater weight of Blood, than on the Muscles belonging to the Hands and Arms; and yet the latter, are far more agile and nimble in Motion.  
Again, if we but make a Ligature two or three fingers breadth above the Cubit, so as to hinder the Reflux, but not the Influx of the Blood; when the Sanguiferous Vessels are swelled, and the Muscles well filled, they are rendered much more unapt for Motion: Which evidently shows that the more the Sanguiferous Vessels are distended, the more the Motion of a Muscle is hindered; because when the Vessels are dilated they resist the Intumescence of the Carnous Fibers, and consequently the Contraction of a Muscle.  
Furthermore, That the Contraction of a Muscle is not performed by the Pondus of the Blood, is apparently evident in the following Experiment: 
A Fifth.  Viz. Make a Ligature two or three fingers breadth above the Cubit, so strong as to stop the Circulation of the Blood; then open the largest Vein with the common Instrument, and the Blood which more than usually swelled up the Sanguiferous Vessels will leap forth; but presently for want of subsequent Matter, and due Circulation, the Vein falls and the Blood ceases to run; yet by a Contraction of the Muscles to which that Vein leads, forthwith part of the Blood, which lay in the Muscle, is forced out: which plainly shows there is less Blood in a contracted Muscle, than before Contraction.  
The same appears also in Running, 
A Sixth.  or any other violent Exercise, in which most of the Muscles are Contracted; for presently the Blood flows more than usually upon the tender Lungs, and causes an Intolerable shortness of Breath: Because by an Intumescence of the fleshy Fibers, the Muscles are not only made incapable of receiving so much Arterial Blood as before Contraction; but also, because the Blood which lay in the Veinous Ducts is driven out more forcibly upon the Heart, and the Heart being overwhelmed, drives it out upon the Lungs. Whence proceeds that Palpiration of the Heart upon violent Motion, not directly as Dr. Croone supposeth, because an Instinct is sent from the Brain to raise stronger Pulsations, that the Blood may forcibly be driven into the Muscles, and thereby cause Contractions; for then why doth it not at the first instinct thus oppress the Lungs with its quantaty? but this strong Palpitation is Accidental; for when the Blood is more than usually driven upon the Heart, and not so plentifully received by the Muscles; by degrees it oppresses the Heart with its quantity, and the Animal powers being sensible of the oppression, raise stronger Systoles to drive it out which presently is received and heaped up in the Lungs as a Part less able to resist its violent Incursion. Nay many times, when the Vessels are well filled with Blood, by violent and frequent Exagitations, betwixt the Contraction of the Heart and of the Muscles, its texture is so loosened; that it ferments and boyles up so that the Lungs are not only swelled up and almost stifled, being rendered uncapable of receiving Air, but it is also more strongly & forcibly driven into the Muscles themselves, and there distending the Sanguiferous Vessels hinders the Intumescence of the Fibers, and thereby the Contractions of the Muscles So that I have not once, trying this Experiment, found such an Oppression on my whole Thorax; and such contrary and in some measure painful endeavours, betwixt the Sanguiferous Vessels tumified and the Carnous Fibers striving to swell; that I was forced to lay me down, to the end that the Carnous Fibers being flaccid might give way to the incourse of the Blood; and by receiving it more plentifully might case both my Thorax of its Oppressions; and that the Carnous Fibers yielding to the Sanguiferous Vessels might put an end to that uneasy contention. Thus much I think is sufficient to prove, that the Blood as a Pondus hinders the Contraction of a Muscle.  
Dr. Ridley supposes, 
Dr. Ridley's Opinion examined.  That the Nervous and Carnous Fibers are only a Congeries of Fluids' contained in certain Vessels; and that by Reason of a Plenitude in the aforesaid Vessels, the whole Machine is in a constant AEquilibrium, so that it will follow, upon the common Postulatum, viz. That the Sensitive or Rational Soul can command the Animal Spirits into a primus impetus; and that part of that Liquor, whenever a Muscle is Contracted, is transmitted through the Vessels from the Brain to its Carnous Fibers, and causes the Intumescence of a Muscle; the same Liquor at the same time being driven back with an equal speed from the Antagonist Muscle, into the room of the former, which was transmitted from the Brain to the Contracted Muscle, to avoid a Vacuum.  
That the Nervous Ducts as well as Carnous Fibers are always Watered with a Fluid; 
The common Postulatum is not to be allowed.  and that the Nervous Liquor equally flows into the Branches of all the Nerves, according to their Proportion, is beyond Contradiction; but if we allow the common Postulatum, we must also conclude, That the Animal Fluid is Intelligent, and there will follow the same Difficulties as were before mentioned, about the leaping forth of Spirits out of Dr. Willis his Tendinous Receptacles; where I have given sufficient Reasons to reject this Postulatum as Impossible.  
Moreover if it were Possible, 
There is no Retrocession of a Fluid from the Autagonist.  for so much of this Animal Fluid as is requisite to distend the Carnous Fibers, to be conveiged in so short a time as Muscular Motion follows our Appetite through the small Branches of the Nerves; yet it would be altogether unconceivable, how it should flow back from the Antagonist, thus contrary to the Course of Circulation; but the Reason he gives is partly this, to avoid a Vacuum: This is a most Stupendious sort of a Motion; at the same time that the Liquid is forced into the one Muscle, it runs directly opposite from the Antagonist; which is very irregular and unlikely.  
Besides, I cannot understand how it comes to pass, 
Nor is it forced out so as to cause a Vacuum.  that the Animal Fluid is so much forced out of the Branches of the Nerves leading, to the Muscles to be contracted as to leave a Vacuum for the Reception of Liquids from the Antagonist; but if an Emptiness or Vacuum could be so made, the Fluids would as soon be drawn back again, out of the Muscles into which our Appetite directed them, to fill that empty Space, as out of the Antagonist.  
To prevent an Objection against his Supposition, that the fluid runs back again from the Antagonist, he thinks he has answered it already, but very insufficiently: He says, if it be said, that the Reflux is opposed by the constant direct Motion, it's easy to reply, that it's slow direct Motion, is easily repelled, by the violent impulse of the forcibly relaxed Muscle. I must confess, it's no piece of difficulty to make such an Answer; but the Reply seems not so easily to give satisfaction; I would fain know how the Antagonist comes to be so forcibly relaxed, as by a violent impulse to overpower the direct Motion: For the direct Motion cannot be so weak, as to be easily overcome; and since the Animal Fluid, as he calls it, is continually driven by the Pulsation of the Heart, through the Brain or Spinal Marrow into the Nerves, whatever repels the direct Motion, must be of equal, if not greater, force than the Systole of the Heart.  
He begs leave to ask, how when another bends his Arm against his Will, the Muscles become tumid, as when voluntary contracted. This Question doth not at all confirm his Hypothesis, but on the contrary pleads against it. It doth not confirm it, because he doth not account for the Phaenomenon, nor give a Reason agreeable to his Opinion, but leaves the thing barely without explanation; it pleads against him, because according to his Opinion, the Muscle instead of growing tumid, ought violently to relax, at least to endeavour a relaxation, and not contrary to the Appetite, to run into a Muscle against our Will: For he says at the same time, that the Muscles to be contracted, grow tumid; the Antagonists by a violent relaxation, drive out the fluid contained in them: So that it will follow, that at the same time we endeavour to keep our Arm unbent, by striving to keep the one Muscle contracted, at the same time we ought to endeavour a relaxation of the Antagonist; and though that force which bends our Arm, doth resist the Voluntary Contraction of the Muscle, yet there would nothing hinder the voluntary Relaxation of that Muscle, which is the Antagonist, but the slow direct Motion, as he calls it.  
But here I humbly take leave to ask, whether there is not a great deal of difference, betwixt a Muscle that is thus tumid, and one that is Contracted? For although a Muscle cannot but be tumid, when contracted, yet it may be tumid though not contracted: I mean, as it is in Muscular motion. I have indeed oft taken notice, that when my Arm has been bend against my Will, the Muscles become tumid, but not as in Muscular Motion; for when a Muscle is Voluntarily contracted, it's hard and tumid; but when my Arm is bend against my Will, it's far more soft; and though tumid, yet very little contracted. Since the Reason why the Muscles grow thus tumid, cannot be accounted for by this Hypothesis, I shall explain it hereafter.  
Having thus examined, and I hope confuted the aforementioned Opinions, I shall in the next place briefly consider the Fabric of a Muscle, the immediate Instrument of Motion.  
Anatomists, describing the Fabric of the Muscles of the Body, divide 'em into Simple, Compound, and more Compound; as also every Muscle into Three Parts, the Head, the Belly, and the Tail; they acquaint you how the Belly is composed of several Fasciculi of Fleshy Fibers, which lie parallel to each other, etc. of which enough may be seen, with a full Description of each sort of Muscles in the Original Authors: I shall omit the Repetition here, and only take Notice of its Fabric, as far as relates to my Hypothesis.  
Every Muscle is endowed with opposite Tendons, 
Of the Structure of a Muscle.  which are composed of several Fasciculi of Fibers, lying parallel to each other.  
These Fibers being strictly joined together, compose a strong, tenacious and firm Tendon; but being separated one from another, are more fragile, and subject to be easily broke, and pulled in pieces; they are continued from each Tendon to its opposite.  
The Interstices of the Fibers thus separated, are filled with the Branches and Extremities of Vessels, the major part of which are Sanguiferous; so that the Fibers continued through the Belly of a Muscle, being obscured by the colour of the Blood, seem to degenerate from the Tendinous Fibers, and therefore are called Fleshy.  
These Fibers as they are continued from each opposite Tendon, so also there is a continued Cavity from one end of Every Fiber to the other; which being partitioned by several transverse and perforated Membranes, every Fiber, according to Dr. Croone, resembles a continued Series of Bladders opening one into another.  
By an Intumescence of these Fibers, they are contracted in length; 
The formal cause of the contraction of a Muscle.  and by a Contraction of their Longitude, each Tendon is drawn nearer to its opposite; which is the formal cause of Muscular Motion, as far as can be made evident by autopsy: This is so unanimously assented to, and so apparent to the naked Eye, that it requires no further proof.  
As for the Efficient Causes of Muscular Motion; before they can be sufficiently shown, we must inquire how many sorts of Vessels are implanted into each Muscle, which according to Anatomists are Four, Nerves, Arteries, Veins and Lymphducts; 
Vessels implanted into a Muscle.  the Nerves and Arteries furnish the Muscles with Spirits and Blood; the Veins and Lymphducts, carry back the superfluity of Blood and Lympha.  
The Branches of all these Vessels are minutely distributed through the whole Muscle, 
Their distribution.  and by an intermixture and texture of the most Minute and Subtle Branches of the same, is form that Substance, which fills up the Interstices of the Fibers, and which denominates the whole Belly of a Muscle Fleshy. The colour of these Vessels, as of all others, is without doubt White, but by the Contents of the Sanguiferous Vessels, the whole Belly of a Muscle is coloured Red.  
The Terminations of these Vessels are so minute and small, so brittle, and subject to be broke, that it's as difficult to trace, and find their Terminations, and Insertions, as to divide a Hair into an Hundred Parts; and though many things are made visible by the help of Microscopes, which are otherwise too subtle to be discerned and discovered by the naked Eye; yet I am not without Reason persuaded, that the terminations of these Vessels, cannot be perceived by the help of a Microscope.  
Some indeed, whatever they think is probable, lest it should not be credited, presently strenuously affirm, they saw it with their Microscopes; and so easily impose on those that less inquire after truth themselves, but take it on trust from others; and perhaps prevent those not so tightly skilled in Microscopes, from making any further search after truth. I shall not here say, that what I propose concerning the Terminations of these Vessels, was seen with any Microscope, but shall declare my Opinion in this Matter, too subtle for any Microscope but Reason.  
It being impossible for any one to understand the true Reason of Muscular Motion, 
And terminations in small Glands.  unless first acquainted with the distribution of these Vessels, and their Contents; first, I conceive, that as the Nerves, Arteries and Veins, are all distributed through a Muscle, so it is but Reasonable to conclude, that their minute and subtle twigs are all, before they terminate, interwoven and mixed with one another: And if so, it will not be absurd to imagine, that they all end, as it were, in small and Diminutive Glands: And if the ends of the Vessels be so small, that their terminations cannot be perceived by a Microscope, these Glandules composed of the Invisible ends of the aforesaid Vessels, must needs be very small and fine, and consequently may not be perceived by the help of extraordinary Microscopes.  
From these Glandules thus form, are derived short Ducts or Passages, for the conveyance of a subtle Liquor, out of the aforesaid Glandules, into the Cavities of the Carnous Fibers. This Passage very likely, may be composed of the external Arterial Coat. Into the Cavity of the Carnous Fibers, besides these Ducts, the twigs of the Lymphducts are also inserted, to carry back a superfluous Lympha.  
If any one thinks it improper or absurd to assert, 
Proved.  that these Vessels do thus terminate, as if it were in small Glandules; let him but consider what a Gland is, and how it is composed, and he will not think the Conclusion irrational.  
For a Gland is nothing else, 
What a Gland is.  but a Composition of an Infinite Number of small and diminutive Vessels, several, and most of which, have Communication one with another; which Gland so composed, is as a Storehouse to receive and contain a certain Liquor, till necessity requires, or its quantity causes an exclusion of it: And this seems plainly to be favoured by Dr. Ridley, when he says, that the whole Body is a continuity of Vessels, ad infinitum. Now, if a Gland consists of an infinite Number of small Vessels; and if the Branches of Nerves, Veins and Arteries be divided before they terminate, into an innumerable number of most invisible twigs, as is most certain; when from one twig of each sort, is derived a vast number, and all these so derived, are mixed and interwoven one with another; why may we not say, at least Metaphorically, since they in their Distributions imitate the Composition of Glands, that they terminate in small Glandules.  
Some perhaps will ask, how must we discover these Glands Malpigius hath observed the Liver to be composed of Clusters of Glands, hanging at the Extremities of the larger Vessels, like so many Bunches of Grapes; and the Glands in other parts are apparent; but in a Muscle we can discern no such things; for it, especially when boiled, parts into distinct Carnous Fibers.  
To which I Answer, 
Why they cannot be discerned.  That those Glands are nothing else but a Composition of the ends of importing and exporting Vessels; and the Reason why we cannot discern the same in a Muscle is this, because the Extremities of the Vessels are so fine, and consequently so brittle, and are contained in the Interstices of so strong and firm Fibers; that whenever we endeavour a separation of the Fibers, the Glandules are torn away from the Vessels from which they are derived; whereas the Fibers which run transverse the Liver easily crumble away, being much more weak and less Numerous.  
The Reason why the Muscular Fibers are so much stronger, than those that run through the Liver, is apparent enough, Viz. because the former are the Instruments of Motion, and consequently there lies a greater stress of them, than on the latter, which serve to make the Part a little more compact, and to keep the outward Coat from being loose upon its Circumference.  
Now because it is impossible to separate and remove these Muscular Fibers, so as to leave the Sanguiferous Vessels as entire as would be requisite for the exact and certain discovery of their Terminations; and because it is certain, that these Vessels are divided, and subdivided ad minimum; that they are interwoven one with another, and that a Gland is nothing else, but a Composition of Vessels; we may rationally conclude they all Terminate in Diminutive Glands.  
The Nerves, Veins and Arteries, composing and ending in these Glands; I shall next Inquire, to what end they do all thus meet together.  
The Use of the Nerves in these is the same as in other Parts, 
The Use of the Nerves.  Viz. to convey a Spirituo-saline Liquor from the Brain: Which Spirits being Volatile and apt to Ferment, when mixed with a Liquor of a contrary Nature, are continually sent by the Systole of the Brain into all the Muscles of the Body, and being laid down forcibly in the aforesaid Diminutive Glands, 
And Arteries inserted in the Glands.  do there meet with Arterious Blood driven in by every Contraction of the Heart: Which two Liquors strongly fermenting together, the Animal Spirits do by an Attrition Subtilise, Attenuate, and Rarify those Particles of the Succus Nutritius supplied from the Mass of Blood; by which Means their own are much dulled, and become less Active for the future; and by a Mixture of these Animal Spirits, with the rarified Succus Nutritius, results a Liquor Different from, 
To prepare a Liquor different from the Spirits or Succus.  and of a middle State betwixt, the Animal Spirits, and the said unfermented Succus Nutritius; which Liquor thus compounded and prepared, is driven by a Succession of Matter through its proper Passages, into the Cavities of the Fibrous Cells, the remaining and more crude Part of the Succus Nutritius being received together with the Blood, and carried back to the Heart.  
This Liquor after this manner conveyed to the Cavities of the Fibers, 
The Use of it.  does as it passeth, and is thrust through according to the continual Laws of Circulation, give nourishment to the Parts, and is thence received by the Lymphducts.  
Thus I conceive it is continually prepared in the Glandules, and Circulates through the Carnous Fibers of all the parts of the Body, whilst Motion ceaseth in a moderate quantity; so that the Lymphducts are capable of receiving it, and thereby prevent the Preternatural Repletion of the Fibers.  
To prove what I have hitherto said, besides the Verisimilitude the thing carries with it, I could bring many Arguments to illustrate, and to make it appear more Plausible and Evident; but for Brevity's sake, and to prevent unnecessary Repetition, I shall omit 'em here, because they are to be Mentioned hereafter.  
I have given a brief Account of the Structure of a Muscle, as far as relates to my Hypothesis, and the Use of the Parts, as they serve to Nutrition. I now proceed to the Efficient Causes of Muscular Motion, both Voluntary, and Involuntary; and first of the Reason of Voluntary Motion.  
That Voluntary Motion does depend upon the Dictates of the Soul, and is the Result of its Faculties, but more immediately of our Appetite, is sufficiently Evinced; because it's instantly performed according to our Will. Where the Soul is lodged; how, and after what Manner, it Operates; and what it is) is most difficult to determine and is not designed for our present enquiry: But as it comes under the Consideration of the Brain, and its appendages; I shall defer my Thoughts of that, till I have a further Opportunity.  
In the mean time, That neither the Soul, nor any of its Faculties are the immediate Causes of Muscular Motion, but Operate by the Mediation of the Animal Spirits, will appear from what follows concerning the Reason of Voluntary Motion: And that the Office of the Will, is only to open and shut the Pores of the Brain, by an Organical Motion, as necessity requires, what this Motion is, and how it is performed, and after what manner the Dictates of the Soul and of its Faculties are put in Action, is reserved for its proper Seat.  
After what manner soever the Soul exerts its Faculties, and directs that Subtle Matter the Animal Spirits (which is sent through the Branches of the Nerves, in greater or less quatities according to our Appetite) to this or that Part: it is allowed, that when our Body ceaseth from Motion, and all our Voluntary Faculties are at quiet, the Pores and Passages of the Brain leading to the Instruments of Voluntary Motion are locked or closed up: And then I suppose, that the Spirits moderately flowing into each Muscle, do prepare so much of the Subtle Liquor aforementioned as is sufficient to Nourish the Parts as it passeth through 'em.  
But when the Pores of the Brain are opened, 
The Reason of Tonick Motion  and unlocked, the Animals Spirits forced by a continual Succession of Matter, flow into all the Branches of the Nerves more plentifully, and being laid down in the Glandules, raise a stronger Fermentation; by which means a greater quantity of the Subtle Liquor is prepared, and forced into the fleshy Fibers more copiously and rather faster that it can be received and evacuated by the Lymphducts; so that the Fibers being all equally swelled with its quantity, consequently Contract the Muscles; which is the Cause of a Tonick Motion.  
When these Animal Spirits, are by a pressure and Systole of the Brain and succeeding Spirits, sent yet more plentifully into the Nervous Channels leading to the Muscles to be Contracted, than into those which are in Motu Tonico, or into the Antagonists (the Passages and Pores leading to those Nerves being yet more expanded and opened) there is a greater quantity of the Subtle Liquor prepared in the Glandules, and thrust out into the Carnous Fibers; and their Cells being more swelled and dilated, consequently there follows stronger and more violent Contractions of the Muscles; whence proceeds Local Motion.  
By the Systole of the Brain I don't mean any other Motion than what proceeds from the Pulsation of the Arteries distributed through that Part. That this motion only proceeds from the Pulsation of the Arteries implies, that it is stronger, & drives the Spirits through the Brain into the Nerves with greater force than if it were really the Motion of the Brain itself; the Heart from whence the Pulsation of the Arteries proceeds, being a stronger and more compact Part as to its Substance, than the Brain.  
If it be asked, why this Subtle Liquor is not more easily forced into the Venous Ducts, than into those that lead to the Fibrous Cells, because the former are larger?  
I Answer, That the Venous Ducts are so proportioned, that they might not be capable of receiving all that is laid down by the Nerves and Arteries; on purpose, that some Subtle Parts might be continually driven into the Fibrous Cells; and when by a more copious Influx of Animal Spirits, a stronger Fermention is raised in the Glandules; presently the Contents swell up and are expanded; so that the Veins being not able to receive them as before, more of that Liquor is not only subtilised, but forced into the Fibers.  
As soon as, according to our Appetite, the Pores of the Brain, which convey Spirits to the Nerves leading to the Contracted Muscles, are shut more closely up again, and leave of to be dilated; the Animal Spirits cease to flow more into the Musculous Glandules, than are required to Subtilise, and prepare a sufficient quantity of Nourishment for the Fibers (as in the Antagonist,) then presently the Contraction ceaseth to be carried on, and the Superfluous Liquor heaped up in the Fibers, is evacuated by the Lymphducts.  
Here perhaps, because I said in the foregoing Paragraph; 
Objections Answered.  the Liquor heaped up in the Fibers is Evacuated by the Lymphducts; it may be a Question whether this Liquor does all the time a Muscle is Contracted, constantly flow through these Cells into the Lymphducts; or whether it remains in the Cells till Motion is to cease, and is not evacuated by the said Ducts till then.  
I say it does continually run of by the Lymphducts, constantly all the time a Muscle is Contracted: Otherwise since as long as Spirits are directed, in a competent quantity, to the Contracted Muscles, to keep up the Dilation of the Fibers; they would presently be incapable of receiving it, and it must needs Regurgitate as it was prepared, and in a great Measure disturb Circulation. But to prevent many absurd Consequences, if it should remain in the Cells so long; I affirm, that it is constantly forced forward, by a Succession of Matter; and as it is forced into, and Circulates through, these Cells, in greater or less quantities, so Contractions are stronger or weaker or not at all, the Fibrous Cells being accordingly Distended, Dilated and Contracted in Longitude; and when according to our Appetite, this Liquor ceaseth to flow into the Fibers in so great a quantity as to Contract the Muscle; that matter which before dilated the Fibers, is Evacuated, as being Superfluous Nourishment, so that for the future it runs through in a moderate quantity as before Contraction, till their Repletion is again renewed to Reiterate Motion.  
If it be asked, why this Liquor is not as easily forced backwards out of the Fibrous Cells into the Glandules from whence it came, as into the Lymphducts; and so think there would be no need for Lymphducts.  
I answer, That if there be no Valve at the inward Orifice of every Dust, to hinder its reflux; (which would be no absurdity to conclude; for we see always upon the like occasion, Nature hath furnished such Places with Valves;) yet I say, if there were none, the direct Motion of subsequent Matter from the Glandules, would sufficiently hinder the reflux of it; and since its reflux is opposed, by new matter forced successively after it; and since according to the course of Circulation, the Matter driven into the Lymphducts before it, will give way for its expulsion, it would be more reasonable to imagine, that the matter contained in the Fibers is evacuated qua datur porta, rather than qua non datur; for when we design the Relaxation of a Muscle, though the Liquor successively sent from the Glands, does not press forwards altogether as forcibly, and in as great Quantities, as whilst Contraction is continued; yet it follows moderately, so as to oppose the Retrocession of any thing; I mean moderately, as to its quantity; for the continual violent concourse of the Animal Spirits, and Arterial Blood, drive it then into the Fibers, almost as violently as when the Muscle is contracted.  
That these Lymphducts do thus receive this Subtle Liquor; 
The Use of the Lymphducts.  and that for that end they are inserted into the Fibrous Cells, I am persuaded; because it cannot otherwise be understood, how the Matter that stuffs them up, and causes Contraction, could be drawn out again, without a Retrocession through the same Channels by which it was brought in: And though the most Learned and Famous Dr. Willis, Dr. Ridley and others, do allow a Retrocession, yet since it contradicts the course of Circulation, we must either deny Circulation, or reject their Opinions.  
Besides, I cannot conceive, what other occasion there is for Lymphducts in a Muscle; since whatever is brought by the Arteries, might as well be returned by the Veins, as by two sorts of Vessels: But for as much as Nature hath made nothing in vain, and since there is so great necessity, that they should be inserted into the Cells, I think it not irrational nor absurd, to conclude they were designed for the aforementioned end.  
That we may understand a little better the Nature of this Liquor, 
The Nature of the Liquor considered, and the manner of its preparation.  which we have so often mentioned; let us consider a little more fully, how that Fermentation is carried on in the said Glandules. It's commonly known, that when a Ferment is put to a Mass, whether Liquid, or of a solid consistence, whatever is the nature of the Ferment, it, according to its power, always endeavours to render whatever comes within the Sphere of its Activity, as like it as possible. 
By Fermentation.  So in these Glandules, the Animal Spirits fermenting with the Arterial Juice, and the Nature of 'em being, according to Dr. Willis and Diemerbroeck, Spirituo-Saline, composed of Volitile Salt and Sulphur, they by Reason of their absolute Quality, more readily mix with the crude Sulphureo-saline Particles of the Nutricious Arterial Juice; and as they are more or less in quantity, so they exalt and carry forth into a Flux, more or less of that Crude Sulphureo-saline Mass; which, being by Fermentation subtilised, is diluted with a sufficient quantity of Lympha, or the watery part of the Serum of the Blood, and the Liquor resulting from that Composition, is thrust out into the Fibrous Cells.  
That this Lympha might be thin enough to dilute the said Liquor sufficiently, it is much attenuated in that Fermentation.  
That there is such a Fermentation, 
Proved.  that the Nerves are inserted into the Glandules, and lay down Animal Spirits, as the Principal Cause of that Fermentation; that the Arteries lay down Blood together, with the aforementioned Juice, and that in those Glands, a subtle Liquor is prepared, being composed of Animal Spirits, and the most? rarified parts of the said Succus; that neither the Arterial Juice, nor the Animal Spirits simply can cause the Contraction of a Muscle; but furthermore, that it is necessary that this Subtle Liquor should be so prepared and composed, as being capable to enter into the Fibers, and to contract the Muscle, is sufficiently apparent from what hath been said, and will more clearly be evinced by what follows.  
And first, if there be a Communication of these Vessels, as certainly there is; than it must needs follow, that there is a commixture of their Contents; if there be a commixture of their Contents, than there is a Fermentation, and in that Fermentation it must needs follow, that the Animal Spirits will according to their Energy and Activity Subtilise and Attenuate the Crudest Parts of the Arterial juice: By a mutual Fermentation there must needs be employed an intimate Commixture, and the Result of that Commixture must needs be different from the Animal Spirits, or Arterial juice simply before they are mixed; so that the Liquor, resulting from the Mixture of 'em, may be properly called a Subtle Liquor: And accordingly as the Animal Spirits are more or less in quantity, to mix with the Arterial juice, more or less will be prepared and thrust out into the Fibrous Cells, and consequently the Fibrous Cells must be more or less Tumified, and the Muscle Contracted, as more or less Spirits are laid down in the Glandules. So that all I have to Prove is, That neither the Arterial juice, nor the Animal Spirits can cause the Contraction of a Muscle, and then it will follow consequently, that there is a Communication of these Vessels, and for that end, in the aforesaid Glandules.  
It hath often been found, that an Atrophea of the Parts will follow the Relaxation; 
The Arterial juice simply cannot cause the Contraction of a Muscle.  & a Palsy the Abscission, or Ligature of the Nerves leading to those Parts: Which sufficiently shows, that the Arterial juice itself, cannot cause the Contraction of a Muscle, without the assistance of the Animal Spirits; because when the Animal Spirits cease to flow into the Glandules, for want of a due Fermentation, to Subtilise and Prepare a sufficient quantity of the Arterial juice, the Carnous Fibers are not only deprived of necessary Recruits, and Alimental refreshment, so that they must needs waste away and grow Languid, but also for want of a sufficient quantity of the Subtle Liquor, to distend the Fibrous Cells, they become destitute of Motion.  
That the Animal Spirits simply, 
Nor the Animal Spirits.  cannot cause the Contraction of a Muscle is proved; because by a Ligature on the Descending Trunk of the Aorta, the Inferior Parts become destitute of Motion: For tho' the Influx of the Animal Spirits, be not hindered; yet for want of Arterial juice, the matter requisite for the Composition of this Subtle Liquor, is not laid down in the Glandules; and the Muscles, the Instruments of Motion, cannot Contract without an Efficient cause.  
From what is contained in these two Paragraphs, it appears; that neither the Animal Spirits, nor the Arterial juice, simply can cause the Contraction of a Muscle; because when the Animal Spirits are hindered to flow into the Glandules, to Subtilise and prepare the Arterial juice, it is not thin enough to be driven out into the Fibers: And tho' when the Influx of the Arterial juice is hindered, the Spirits are permitted to flow in; yet by Reason of the small quantity of 'em, they run through the Fibers without distending 'em.  
That this Arterial juice ought not, 
Because the Arterial juice cannot pass into the Fibers till prepared by the Spirits.  nay cannot, pass into the Fibrous Cells, till subtilised and prepared by the Fermentation in the Glandules, is Plain; because if it could, they would be stuffed up with the quantity of it, driven in by the continual course of Circulation, so as to Contract the Muscle against our Will; which is otherwise.  
Now since the Arterial juice cannot pass into the Fibers till Subtilised; and since the Animal Spirits are not enough in quantity; moreover since neither of 'em, simply are capable of Contracting the Fibers, and if either be obstructed the Action is abolished; I say since it is an Action, that depends upon the mutual Assistance of each; it is apparent. That it must be performed by a Liquor resulting from a Mixture of both.  
And then we must of Necessity allow a Communication of these Vessels; else there could be no mixture of their contents; and the Arterial juice must be prepared before it can be forced into the Fibers, for the Reasons beforementioned: to which end it will be necessary that the Vessels terminate in the Glandules, That their Contents may be mixed, and that this Subtle Liquor (resulting from a Mutual conflict of the Animal Spirits subtilizing the cruder Arterial juice by Fermentation, in greater or less quantities accordingly as the Spirits are laid down by the Nerves) may be prepared and made ready to be driven into the Fibers.  
To promote the Fermentation, and the Preparation of this Subtle Liquor, the forcible influx of the Animal Spirits, and of the Arterial Blood much conduce: For be the Temper of the Arterial juice and of the Animal Spirits never so good, unless they are driven violently one against another, there would be required a longer time before they could be so mixed and, fermented as they ought, but the succession of new Matter admitting no long delay, it is requisite that they should be thus forcibly driven in together; to the end that the Spirits might be diffused through the whole Mass sooner, and that their Minute Particles, striking more violently against the fixed and compacted Particles of the Blood, might more immediately break and separate the strict Union of its Parts; and by a violent Fermentation, sufficiently dissolve and volatilize 'em, in so short a time as the Continual Pulsations of succeeding Humours admit.  
The violent concourse of the Blood and Spirits, do not only promote this Fermentation, and the separation of the Subtle Liquor, but also successively force the Matter prepared and separated into the fleshy Fibers: This is so highly Probable, that it needs no Arguments to make it more Evident; it being the unavoidable consequence of a continual Circulation.  
Now I have given the Reasons of Voluntary Motion, and the Efficient Causes of the Contraction of a Muscle; it will be easy to unfold the Reason of Involuntary Motion. That Voluntary Motion depends upon the Will is manifest; and that Involuntary Motion is not at all Subject to it, but is performed not only without the Dictates of our Appetite but against 'em, is so evident, that it would be superfluous and useless to dispute it: I shall only show how it is performed, which is indeed a little more obscure.  
As Voluntary Motion is performed, 
The Reason of Involuntary Motion.  for as much as the Pores of the Brain are dilated according to our Appetite, so that the Animal Spirits flow more plentifully into the Glandules, and prepare a sufficient quantity of the Subtle Liquor to distend the Fibers as it passes through 'em; so those Pores, which lead to the Nerves serving to Involuntary Motion, are proportioned in such a manner, that they continually convey Animal Spirits in a sufficient quantity to cause a perpetual Motion.  
These Pores being so proportioned, there is no need that they should have such an Organic Motion as those serving to the Instruments of Voluntary Motion; because since they are proportioned so, as to carry Spirits in a competent quantity, there is no necessity that they should be either Dilated or Contracted.  
To Prove that these Pores do lay down a competent Quantity of Spirits to be carried to the Glands, 
Proved.  I need not bring many Arguments; for its plain and evident to all Practitioners in Physic, that when the Spirits are too much carried forth, and exalted, as in a Diary Fever, the Pulse is strong and vehement, and by a too great exagitation of them, it becomes more quick and frequent; it is also Evident, that when the Animal Spirits are depressed, the Pulse is Weak, Slow and Rare, as most commonly in Women, subject to Hysterick Fits; and when the Spirits by an Acute or Cronick Distemper are worn out, and almost spent, the Pulse is either Vermiculans, Formicans or Tremens; which are signs that Nature is almost spent, and ready to yield to the Distemper. I say this is evident enough; it is then beyond Contradiction, that a healthful Pulse depends upon a moderate Quantity of Animal Spirits, and that they continually flow into those Nerves leading to the Instruments of Involuntary Motion; and as their Quantity varies, so more or less of the Subtle Liquor is prepared to distend the Fibers of those Muscles.  
When the Spirits are weak, or almost spent, there must needs flow a less quantity into the Glandules; and when they are exalted, a greater; and if Extremes either frustrate Natural, or cause Preternatural Effects, we may not only conclude, that Natural Actions are performed by a moderate Quantity, but from hence we may bring good Arguments for a further proof and confirmation of my Hypothesis about Voluntary Motion.  
Since I have said, that the Spirits, 
How it comes to pass, that the Systole of the Heart, is interrupted with such frequent Diastoles.  by Reason of a particular Proportion of the Pores, flow continually in a just Quantity; and that these Pores are not subject to be Dilated or Contracted according to our Appetite; it remains, that I should show how it comes to pass, that the Systole of the Heart is interrupted with such constant Diastoles; whereas, when the Pores serving to Voluntary Contractions are open, so as to convey a sufficient quantity of Spirits, the Motion is continual and without Intermission.  
The Reason of the Difference will easily appear, if we do but take Notice, how when a Vein is opened, the Blood runs out continually, without ceasing or intermission; but if an Artery be opened, it gushes out with Intermissions answerable to the Diastoles of the Heart: because the Veins are always as a common Cistern, distended with Blood; and before they can be emptied with a small Orifice, they are filled by the Extremities; but the Blood being continually received by the Extremities of the Veins, and those Extremities being furnished with Valves, to hinder it from Regurgitation; the Arteries are always kept more lank and empty, and are not as the Veins distended with Superfluous Blood: So that it only flows out of them, when they are distended upon violent Systoles of the Heart.  
In like manner, that part of the Brain that furnishes the Nerves serving to Voluntary Motion with Spirits, is like a common Cistern full of Spirits; and when Vent is given (the Pores being opened according to our Appetite,) the Spirits are thrust forth in one constant course: But the Muscles serving to Involuntary Motion, being in continual Action, do so drain their Storehouse, as to keep it more flaccid and empty; so that the Animal Spirits, as they are prepared, are continually waved through their proper Pores; and as one Wave follows another, so one Contraction perpetually succeeds, the Subtle Liquor being according to the same Rules prepared in the Glands, and thrust out through the Fibers.  
Some of the Muscles designed for Involuntary Motion, are, in some measure, subject to our Will, so that it lies in our power to retardate or quicken the succession of their Contractions, but not totally to obstruct or hinder them; as the Muscles serving to Respiration: The Reason of which is this.  
The Pores of the Brain leading to the Nerves, which convey Spirits to these Muscles, 
Why the Muscles serving to Respiration, are subject in some measure to our Appetite.  are so framed, that they carry Spirits to them, as to the other Muscles serving to Involuntary Motion, in a competent Quantity; but these Pores differ from those, for as much as these have such an Organic Motion, as to contract or dilate according to our Appetite, we can by a constriction close up these, so as to deny a Passage for Animal Spirits, sufficient to prepare a quantity of the Subtle Liquor to cause due Contractions of the Muscles; till by degrees they increase to such a great Quantity, and distend their Receptacles so long, that those are no longer capable of receiving more; till that force which drives them violently into the Receptacles overcomes the constrictive Faculty of the Pores; and then the Spirits, against our Will, break forth, and flow violently into the Musculous Glands.  
It is easily noted, that after we have holden our Breath a long time, the first Contractions are as if Two or Three were joined together without intermission: I mean so long continued; which is sufficient to prove what I have said of the Muscles serving to Respiration.  
For when by a Constriction of the Pores, the Spirits which ought to flow out, are kept in, and heaped up in their Receptacle, and Two or Three Contractions are by that means hindered, that Receptacle becomes like a common Cistern; and as soon as the Pores are forced open, the Spirits running out, cause long continued Contractions, till that store is spent, and then they wave through, as before the Interruption.  
What I have hitherto said, might be sufficient to prove my Hypothesis reasonable; but to illustrate it a little more, I shall explain some of the Phaenomena about Muscular Motion, and give sufficient Reasons for them agreeable to my Hypothesis.  
Common Experience tells us, that Old People, 
Why old People are subject to a trembling of their Head or Hands.  whose Spirits are flat and weak, are most usually subject to a Trembling of their Head or Hands; the Reason of which is this: The Fermentation in the Glandules is too low and weak, and doth not subtilise a sufficient Quantity of the said Liquor, to keep the Muscles in a constant Motion: The Reason why a sufficient Quantity of Liquor is not prepared, is plain; for though their Appetite and Desire is strong enough, and endeavours to open and dilate the Pores of the Brain; yet when the Spirits are weak, it's a sign few are separated from the Blood; and if few be separated, they cannot flow plentifully into the Nerves, be the Pores never so wide.  
This confirms the Reason I have given, why the Systole of the Heart is interrupted with constant Diastoles: For the Storehouse which supplies it, is always kept empty, by reason of the constant efflux of Spirits; and in Old People, the Storehouse which furnishes the Nerves serving to Voluntary Motion with Spirits, is kept empty; because few Spirits are separated from the Blood; and as they are separated, they are continually spent by a constant distribution of them, to prepare Nourishment in the Musculous Glands: So that when the Pores are opened wider for Voluntary Motion, for want of a sufficient Stock, they cannot flow out in a constant and equal proportion, but as they are separated, they wave through those Pores that are most ready to receive them. The Motion indeed is not interrupted with such perfect Intermissions, because this Storehouse is not so clearly drained; but there is a perfect Remission, because the Stock is not so copious as in Young and Lusty People to yield constant Supplies.  
Moreover, when the Spirits are weak, it's a sign that the Blood is much depauperated, and declines from its Natural state; and then there is more need for strong Spirits to raise the Fermentation in the Glands, and to subtilise the Liquor: Upon which account, when the Spirits are weak, the Disadvantage must needs be the greater.  
Hence may be deduced a Reason, why when our Spirits are low, and almost spent, though our Appetite be strong, we cannot perform strong Actions.  
From hence it may be proved; that there is something prepared in these Glands by the Animal Spirits, which is incapable of entering into the Fibers till prepared: For whether the Spirits be weak or strong, there is a sufficient quantity of Arterial juice laid down in the Glands, tho' it's not made capable of passing into the Fibers, but as it is prepared, accordingly as the Spirits are more or less in quantity, stronger or weaker.  
I have known Women, who seemed healthful, 
Another Phaenomena Explained.  and of a Sanguine Complexion, whose Hands would, when they were about any Moderate Exercise, Tremble as if Paralytic: For the Mass of Blood being a little more than usually depraved, and degenerated from its Balsamic and Sulphureous, into a more crude and Phlegmatic State, was unapt for Fermentation; so that the Animal Spirits being not able easily to prepare a sufficient quantity of the aforesaid Subtle Liquor, to keep the Muscle in a constant Motion, were forced to do it by an unequal Influx: just as an Horse, set to draw too great a Burden, is forced by many reiterated draughts to get it forwards.  
Mr. Cowper Mentions an Experiment, 
A Third.  how by an Injection of Water into the Crural Artery, the Muscles of the Legs renew their Contractions. From this he would infer, that Muscular Motion is performed by the Blood as a Pondus; but tho' his Myotomia Argues, it came from an Inquisitive Author, yet I rather an fully persuaded, that the Blood does not Act as a Pondus, because this Experiment pleads against it; but to avoid a long dispute, I shall only give the Reason of the Phaenomenon.  
Before the injection of this Water, the Branches of the Arteries are full of Blood, and Arterial juice; and when by the mixture of the Water with this juice, it is attenuated and driven into the Glands, faster than it can be received by the Venous Channels, the most Subtle Part is by the force of injection, strained into the Fibers and distends 'em so as to Contract the Muscle.  
Perhaps here it will be objected that if the Water can thus pass through into the Fibers, what need is there that the Nerves should lay down their contents in the Glandules.  
I answer, that the Water is thin and apt to pass through those Ducts than the Arterial juice, which is thick and viscid; wherefore it is necessary, that the Nerves should lay down their contents there, to Attenuate and Rarisie this Thick juice.  
I have seen People inclining to a Dropsy, 
A Fourth.  whose Blood and Serum was much diluted, could move much more nimbly, tho' more seebly, than some of a healthful Constitution, whose Blood and this Arterial juice was thicker and not so much diluted; which doth plainly show, that the thinner the Blood is, and the more diluted the Nutritious juice, the less quantity of Spirits is required to subtilise it, and make it capable of passing into the Fibers.  
It may easily be observed, that those People whose Spirits are strong, 
A Fifth.  and their Arterial juice very thin, are Nimble; but the Contractions of their Muscles are not so durable, as of those, whose Serum is of a thicker Consistence: For tho' in the former more of the Subtle Liquor is prepared, yet by Reason of its Tenuity it's sooner received, and carried off by the Lymphducts; which makes the Contraction shorter, Those whose Blood abounds with fixed Salts & Phlegmatic Humours, 
A Sixth.  which too much dull and resist the Activity of the Animal Spirits, are always slow and unapt for Motion; whereas, if the Blood Acted as a Pondus, they must be more nimble and strong; because the thicker the Blood is the heavier would be the Pondus.  
As Muscular Motion is many ways vitiated, by the fault of the Arterial juice, or of the Spirits, and consequently by the Distemperature of this subtle Liquor; so it is very often depraved, and accordingly as the Mass of Blood degenerates from its genuine and proper Nature, are produced various Diseases or Symptoms of Diseases: viz. Cramps, Convulsions, Palpitation of the Heart, Leaping of the Tendons in Fevers, etc. for a farther illustration of my Hypothesis, before I conclude, I will give the Reasons of these Phonomaena.  
When by an Abuse of Non-Naturals, the Ferments of the Viscera are perverted; 
The Reasons of Convulsions.  and by the faults of the Pancreatic juice and of the Spleen, an Acid or Austere Salt is carried forth into a flux; presently the Mass of Blood is vitiated. The Animal Spirits meeting with this vicious Salt, and fermenting in the Glands, do there cause irregular Explosions of matter, into the Carnous Fibers, subtilised in that Fermentation; whence follow irregular Contractions of the Muscles.  
Why Convulsive Paroxysms come at uncertain times, will easily appear, if we do but consider the Procatartick Causes; amongst which I shall only mention two, viz. The Quantity of Morbific matter irritating Nature to an Expulsion of it; and sudden Passions of the Mind.  
The Quantity of Morbific matter is far greater in some Bodies than in others, before the Spirits are able to recover themselves, and to endeavour an Expulsion of it: In the former Case, Convulsions are Universal and seize the whole Body; in the Latter Particular, the Morbific matter being accidentally driven more on one Part than another. Again in some Bodies it is sooner heaped up, being generated in greater quantities.  
The Matter heaped up, at the first is very crude and thick, and although it be cast forth into, and Circulates through, the Musculous parts, and continually Ferments with the Spirits in the Glandules; yet because it is not sufficiently attenuated to be driven forth into the fleshy Fibers, till by frequent Circulations and Fermentations, it is exalted from its state of sixedness to a more Volatile, no Convulsions succeed. Moreover as long as it continues in its state of Crudity, the Animal Spirits are much dulled and their Activity quashed by mixing with it, but it being at the length subtilised and rarified by frequent Circulations, the Animal Spirits recovering 'em selves, and violently Fermenting with it in the Glands carry this Vicious juice explosively into the Fibers. The Reason why this Motion is Involuntary, is, be cause it is not produced by a greater quantity of Spirits flowing from the Brain, accordingly as the Pores are dilated by our Appetite, but by a mixture of Morbific and Fermentitious Particles, which cause Preternatural Fermentations & Expulsions of Matter Attenuated thereby.  
Convulsive sits are sometimes brought on, before the Morbific matter gradually arrives at this state of tenuity, when upon sudden Passions of the Mind, the Animal Faculties quit their Stations, and being overpowered by external Objects can no longer moderate the Emanations of the Animal Spirits. The Formal Cause of the Irregular Emanations of the Spirits in these Convulsions is this; the Pores of the Brain being shut up, to keep out external Objects, heap up the Spirits, till by Reason of their quantity, the Pores can no longer retain them; and then the Spirits rush out quà datur portà in a greater quantity, and violently setting upon the Crude juice, which they meet with in the Glandules, strongly Ferment Attenuate and carry it forth into the Fibers, of the Muscles wherein the Spirits are thus accidentally laid down: The Brain being thus emptied, the Pores are shut up again, till the quantity of Spirits make way again; and so successively follow Convulsive Motions, in this or that Part wherever the Spirits set upon the Crude Morbific Matter.  
The 〈◊〉 of the Heart, as the 〈◊〉ed Dr. 〈◊〉 has noted, 〈◊〉 proceeds 〈◊〉. Convulsive Motion: The Reason why the Morbific Matter only shows itself in this part, and at the same time in no other parts of the Body in this, The Mass of Blood 〈◊〉 nated with a Morbific 〈◊〉 and whatever is the Nature of it, since it is 〈◊〉 〈◊〉 the whole Body, 〈◊〉 〈◊〉 Rationally expect Universal Convulsions as well as 〈◊〉: But this Morbific Matter being as yet Crude, and not of a sufficient Quantity 〈◊〉 〈◊〉 Nature in all the Parts of the Body; these Effects are only produced in this Part, where a large Quantity of Spirits is continually laid down to serment with it; by which means it is Subdued in this part, before it arrives at such a State of Activity of such a Quantity, as to cause irregular Fermentations and Contractions in other Parts.  
In a 〈◊〉 〈◊〉 〈◊〉 〈◊〉 〈◊〉, The Muscles continue constantly contracted against our Appetite; 
〈◊〉 〈◊〉 〈◊〉 〈◊〉 〈◊〉.  these Distempers seize People, whose Blood is rather of a Vitriolic Tenacious Nature, and impregnated with more Fermentative Spirits; so that by reason of the Viscidity of the Matter violently forced into the Fibers, either the Lymphducts are obstructed, or by reason of a continual Fermentation, Matter gradually succeeds.  
In the Cramp, the Fermentation is different from that which is Natural, 
Of the Cramp.  or in other Convulsive Motions; that which is Natural, being caused by the Animal Spirits preparing a Subtle and Homogenous Liquor: In other Convulsions the Animal Spirits ferment with, and endeavour to subdue a Morbific Humour brought and laid down by the Arteries, and the Matter thus attenuated, is accidentally forced into the Fibers: But these are caused by the continual Conflict of the Volatile Animal Spirits, strongly fermenting with Fermentative and Elastic Particles of another Nature, supplied by the Mass of Blood; where both striving to subdue each other, the Elastic Particles of the Blood, more strongly oppose the Motion of the Spirits, and the Spirits them, by a contrary agitation and intestine Motion of their Particles; and these Two being violently exploded, and impetuously leaping into, and fermenting in the Fibers, cause most strong and painful Contractions.  
The Subsultus Tendinum in Fevers are caused, 
Of the Subsultus Tendinum in Fevers.  when the Spirits being too much exalted, leap irregularly out of the Brain, and raise the Fermentation in the Glands so high, as to prepare and carry too much of the Subtle Liquor into the Fibers; which because it is so very thin, easily slips into the Lymphducts, and makes the Contraction short.  
I shall add the Reason only of one other Phaenomenon, 
Of a Spasmus Cynicus.  which seems more evidently to prove, that Muscular Motion depends on the greater influx of Animal Spirits, viz. Why the Contraction of one Muscle followeth, when the Antagonist becomes Paralytic, as in Spasmus Cynicus: The Branches of the Nerves being derived from the Brain, when the Influx of the Spirits into the Muscles of the one side are hindered, they regurgitate, and are driven presently in a greater quantity into the Antagonist; as by the Loss of one Eye, the other, by a greater Influx of Spirits, becomes more Acute: This is evident, and needs no further proof.  
Thus I have laid down my Hypothesis of the Formal and Efficient Causes of Muscular Motion, and how it is vitiated or depraved; which I the more willingly commit to the Judgement of Learned and Competent Judges, with due Submission; because it seems to me to answer Natural Ends, and to account for the several Phaenomena, without the Difficulties, Enormous Motions and Impossibilities, which usually attend Hypotheses on this Subject; not doubting, but it will give satisfaction, till one more probable and plausible shall succeed.  
FINIS.   

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