ANATOMY. 
more than one third of their extent. The inteftines are 
exceedingly tenacious of their vis infita, as th.ey continue 
to contract themfelves long after they are taken out of the 
body, and even after they are cold ; and the heart is even 
more tenacious of this principle than the inteftines, as is 
evident in a young .chicken, and in cold animals. Differ¬ 
ent mufcles are bed excited'iiito aCtion by different ftimuli; 
as the bladder by urine, the heart by the blood, and the 
inteftines by air. Though the nerves are removed, or the 
connection, made by them between the mufcles and tjie 
brain, cutoff, the mufcles lofe but little of their irritable 
nature. It appears alfo from the example of the polypi 
and other infects, that' the fame irritable difpolition ex¬ 
tends very widely through the animal fibres. In many of 
thefe creatures, neither brain nor nerves can be traced, yet 
they are exceedingly affected by ftimulus. Laftly, we per¬ 
ceive this vis infita in vegetables, many of which expand 
and contraCt their flowers and leaves according to the dif¬ 
ferent ftates of the atmofphere; and fome plants exert 
hidden and violent motions from ftimuli externally applied, 
appearing little inferior in this refpeCl to animals. This 
vis infita does not depend upon weight, attraction, or elaf- 
ticity ; for it is feated in a foft fibre, and vanifhes as the 
fibre grows hard. 
That the caufe of motion is conveyed through the nerves 
into the mufcles, is certain from the experiments made on 
the brain and nerves. For the nerve alone has feeling; 
this alone carries the commands of the foul; and of thefe 
commands there is neither intimation nor perception in that 
part, whofe nerve is either tied or cut, or which has no 
nerve. Moreover, on irritating the nerve or fpinal mar¬ 
row, even in an animal that has been dead for fome time, 
the mufcles which have nervous branches from thofe parts 
are moft vehemently convulfed. When the nerve of any 
mufcle is cut or tied, or the bafis of the fpinal marrow 
compreffed, or that part of the brain from whence the 
nerve has its origin, the mufcle becomes paralytic, and lan- 
guifhes, and cannot by any force be re-called into an aCtion 
fimilar to the vital one. But, if the ligature be taken off 
from the nerve, the force by which the mufcle is put into 
aCtion is again recovered. The nerve being irritated below 
the place where it is cut, the mufcle to which that nerve 
goes is contracted. This appears from numerous experi¬ 
ments, efpecially thofe made on the phrenic and recurrent 
nerves. This force, called vis nervea, is not the fame 
with the vis infita. The former comes to the mufcle from 
.without: whereas the other refides conftantly in the muf¬ 
cle itfelf. The nervous power ceafes when life is deftroy- 
ed ; the other appears, from certain experiments, to remain 
for fdme time after death: it is alfo fupprelfed by tying a 
ligature upon the nerve, by hurting the brain, or by ta¬ 
king opium. The vis infita fuffers nothing from all thefe 
circumftances : it remains after the nerve is tied ; it con¬ 
tinues in the inteftines though they be taken out of the 
body and cut in pieces; it appears with great ftrength in 
fuch animals as are deftitute of brain; that part of the body 
is moved which has no feeling; and the parts of the body 
.feel which are without motion. The will excites and re¬ 
moves the nervous power, but has no power over the 
vis infita. 
In the motion of the mufcles, whether owing to the iris 
infita or the vis nervea, the fibres are contracted towards 
'the middle of their belly, and recede from one another 
outwards; they are alfo diverfified with various tranfverfe 
wrinkles ; tire whole mufcle alfo becomes ffiorter, and 
draws its extremities towards the middle; hence it draws 
towards each other thofe parts with which it was con¬ 
nected ; the mufcle alfo fwells by its contraction, becom¬ 
ing hard at the fame time, and as it were increafes its cir- 
cumference on every fide. Haller has never oblerve-d it 
to turn pale. Whether on the whole it is increafed in bulk, 
or acquires more in breadth than it lofes in length,' is dif¬ 
ficult to be known. It draws after it the tendons, which 
are obfequious to its motions, though of themfelves nei¬ 
ther moveable nor'irritable. The whole mufcle is alfo 
5 6 3 
capable of being moved at once, or only a part of it; if 
one extremity is fixed to an immoveable part, that only is 
moved which is capable of yielding. 
If it be demanded, Whether the arteries conduce any 
thing to mufcular motion? and whether the palfy of the 
lower limbs after a ligature upon the aorta be not an ar- 
guinept of this opinion? we anfwer, No farther than as 
they conduce to the found ftate of the mufcle, or to the 
confervation of the mutual ftrutture and habit of the parts, 
which they fupply with vapour, fat, &c. for the irritation 
of an artery does not affeCt or convulfe the mufcle to which 
it belongs; nor does a ligature on it caufe a palfy, unlefs 
after a conliderable time, when the nmicles begin to be 
deftroyed by a gangrene. Moreover, it is impracticable to 
explain the motion of peculiar mufcles from a caufe de¬ 
rived with an equal force from the heart to all parts of the 
body. Laftly, the influence of the will is confined to the 
nerves, without redding in the arteries or other f'olid parts 
of the body. But the direCt manner by which the nerves 
excite motion in the mufcles, is fo obfeure, that we may 
almoft for ever defpair of its difeovery. The opinion, that 
the nervous velicles fvvell by a quicker flux of the nervous 
fpirits, is inconfiftent with anatomical truth, which demon- 
ftrates the leaft yilible fibres to be cylindrical, and in no 
part vefic'ular ; it is likewife repugnant to the celerity with 
which nmfcular motion is performed; and it Teems to be 
completely refuted by the well-known faCI, that the bulk 
of a mufcle is rather diminiftied than increafed during its 
aClion. Again, the inflation of rhomboidal chains in the 
fibres is - equally repugnant to the celerity of miifcular mo¬ 
tion, and to anatomy; they would alfo occafion an im- 
menfe wafte of ftrength, and after all render the mufcle 
but little fhorter. The nerves want that irritable nature 
which is obferved in the mufcular fibre. Finally, it is by 
no means demonftrable, that the fibres, from fo few nerves, 
can be fo numerous, or diftributed in fo many different 
tranfverfe directions, with refpedt to the mufcular threads', 
as thofe hypothecs require to be allowed. A complica¬ 
tion of the nerves round the extremities of fibres, fo as ter 
contract them by their elafticity, is founded upon a falfe 
ftrutture of the mufcular fibre, fuppofing the nerves to be 
diftributed where filaments of the cellular fubftance only 
can be demonftrated. Moreover, the phenomena of ani¬ 
mals, which, having heither brain nor nerves, -are yet 
very apt for motion, apparently demonftrate the intrinfic 
fabric of the mufcles to be fufficient for their motion; 
without other afliftance from the nerves. Other expla¬ 
nations, derived from fpherules of air in the blood, are 
founded on a falfe hypothefis, for the blood contains no 
elaftic air. The animal fpirits are not of the nature of an 
eleCtric torrent. 
If we may add any thing to thefe phenomena; we may 
fuppofe the nervous fluid to be of a ftimulating nature, by 
which means it forces the elementary particles of the muf¬ 
cular fibre to approach nearer to each other. The motive 
caufe which occasions the influx of the animal fpirits into 
the mufcle, fo as to excite it into action, feems not to be 
tfye foul, but a law derived immediately from God. For 
animals newly born, or newly transformed, without any 
attempt, or exercife, know how to execute compound mo¬ 
tions very difficultly to be defined by calculation; whereas 
the foul knows not how to perform any actions until the 
has learnt, by making experiments, the methods of per¬ 
forming them. A mufcle then is contracted when in a 
given time it receives more of tire nervous fluid, whether 
that be occafioned by the will, or by fome irritating caufe 
arifing in the brain or applied to the nerve. 
But, though we may fuppofe the foul to be the capfe of 
the nervous motion, we cannot do the fame with regard 
to that arifing from the vis infita. The heart and intef¬ 
tines, and alfo the organs of generation, arc governed by 
a vis infita, and by ftimuli. Thefe powers do not arife 
from the will; nor are they leflened, or excited, or fup- 
prelled, or changed, by it. No cuftom nor art can make 
thefe organs fubjeCt to the will, which have their motions 
from. 
