ny 
- to the contrary fide. 
The seg ssi O Ray 
ferved, that the part of the nerve above the ir- 
ritated region, contract alfo thofe higher mufcles, 
that have their nerves from thence. We very 
clearly find alfo by experiments, that the motion 
produced in the mutcle by irritating the nerve, is 
always propagated out of the larger into the 
{maller branches, and goes afterwards continu- 
ally defcending. The nerves defiened for the 
fenfes are circumftanced in a quite different man- 
ner; for in thefe, the fenfitive motions, doubt- . 
lefs, tend upwards. In order to contract any 
mufcle, it is necefflary that its nerve be irritated 
in the region above the mufcle, or at its infer- 
tion into it; fince that motion never tends up- 
wards, but always downwards. 
It may now be probably afked, wherein I 
think the beginning of that natural irritation, fti- 
mulus or provocation of the motion, thus com- 
municated to the mufcles through the nerves, 
confifts? Since I deny, that any vifible flowing 
and inflating fpirits, are locally moved through 
the nerves; and, on the contrary, think that a 
certain ‘inftantancous irritation is much more fub- 
tile, and capable of the effort, than the {pirits 
fuppofed by fome neceflary to move the mutcles: 
and that, from thence, it follows, that this irri- 
tation fhould not only have its origin elfewhere, 
but that a force is likewife wanting, by which 
that motion is transferred through the nerves into 
the mufcles. I confefs thefe things are requifite, 
fince experience demonftrates it even to the eye. 
To give a proper anfwer to this queftion, I 
think, the beginning or principle of that motion 
lies in the fpinal marrow, and is alfo in all the 
nerves of the body ; fo that the marrow, and all 
the nerves, are conftantly and perpetually irri- 
tated to give motion to every mufcle of the whole 
body. I would have it particularly obferved, 
that I admit no effential difference between the 
natural and fpontaneous contraction of the 
mufcles, and that performed by the will. I con- 
fider this difference as merely accidental ; but, 
becaufe we move all thefe mufcles, which we 
move voluntarily, in a contrary direction; that 
what is faid to be effential in the contraction of 
all mufcles, is a natural contra€tion. For this 
reafon, voluntary motion ceafes, or is changed 
in us, as well as in all other animals; when ei- 
ther the antagonift mufcles are wanting, or when 
one of a pair is more powerful than the other, 
as I have formerly demonftrated in my treatife 
on Refpiration. And indeed we cannot move 
mufcles at will, unlefs we have the power of de- 
termining the natural motion of the antagonifts 
But if all the motions of 
our mufcles are continual and natural, when the 
antagoni{ls are wanting, asis the cafe with re- 
{pect to many mufcular parts of our body, which 
we have not power to move at our pleafure; 
only: fo far as thofe mufcles are firft dilated by 
their contents. ‘Thele perform the office of an- 
tagonift mufcles, and give us the power of mov- 
ing them ina contrary dire@tion at the command 
of our will: but otherwife, all things acquiefce 
in the perpetual contraction. 
In order further to explain the origin of this 
natural and perpetual contraction of the mufcles ; 
of: EN jScEsC TS. bag 
I think indeed it arifes from the continual im- 
pulfe of the arterial blood upon the marrow and 
nerves: for, by means of this blood, all thofe 
parts feem to be continually moved, excited, 
and irritated to convey that motion pepetually 
and uniformly to the mufcles, and to prepare 
the latter for their perpetual contra@tion. For 
this reafon, all the nerves without exception, 
have not fewer arteries in proportion, than the 
brain itfelf, and the fpinal marrow have. I 
fhould think indeed, that this matter might be 
eafily afcertained by experiments: for which pur- 
pofe, I once thought of inje€ting a peculiar li- 
quid through an artery into the marrow, by a 
{mall fiphon, and then tq obferve carefully, whe- 
ther any motion was thereby excited in the 
mufcles. But I would again advife the reader, - 
to confider well that wonderful motion and 
- power of the mufcle, when its nerve is in the 
leaft difturbed or ftimulated. 
It is now time to proceed further; and I thall, 
by.a curious experiment, demonttrate to the eye, 
that the mufcle is not at all fwollen in its con- 
traction, or becomes thicker by inflating it, 
and therefore occupies the larger {pace ; nay, on 
the contrary, that its {welling decreafes: and 
therefore, in its contraction. or action, fills lefs 
fpace than when it refts flaccid. I fay refts, be- 
caufe I cannot obferve that the mufcle in the 
living animal, ever abfolutely ceafes from all 
motion. “And therefore it fhould be rather faid, 
that it is lefs ftrongly moved at the time of its 
relaxation; or then only recollects its elaftic 
ftrength, that it may be able, the moment after, 
to make the ftronger effort. to contract itfelf. 
This may be feen very clearly in regard to the 
motion of the heart and auricle in the Frog; 
for the blood is there brought back from the 
circumference of the body, according to the laws 
of circulation, and being driven in to the auricle, 
it may be confidered as the auricle’s antagonift 
mufcle, which dilates it: but the auricle itfelf is 
the antagonift of the heart, fince by means of 
the blood, which it protrudes into its cavity, it 
likewife dilates its fubftance, fo that the won- 
derful, repeated, and continual pulfe of the 
heart, has its origin from this alone: that 
pulfe is therefore perfectly natural and necef- 
fary ; for thofe two mufcles, that is, the heart, 
and its auricle, are unequal in fize and ftrength, 
and therefore their motion is neceflarily varied, 
If the auricle were as firm and as ftrong as the 
heart, the motion of each would abfolutely. ceafe ; 
for, wherever the power of the antagonift is 
equal, there is obferved no motion of the mutcles, 
and all things are in both in equilibrio, until there 
arifes another determination, which, caufing one 
mufcle, to be contracted fomewhat more ftrong- 
ly than another, at length moves our limbs. Such 
a determination may proceed from various caufes, 
If, for example, a man’s fkin be very gently 
rubbed and irritated with a hair doubled feveral 
times; I often obferved, that the motion of the 
antagonit mufcles of the arm and hand was im- 
mediately determined; fo that the perfon inftan- 
taneoufly, asit were, unknown tohim, has put 
his hand to the place where he felt the titillation, 
Ti and’ 
