$6 4 ANA" 
from a vis infita ; nor can they be made to pbey the com¬ 
mands of the foul, like attendants on voluntary motion. It 
is fo certain that motion is produced by the body alone, 
that we cannot even fufpeft any motion to arife from a 
fpiritual caufe, befides that which we fee is occafioned by 
the will; and, even in that motion which is occafioned by 
the will, a ftimulus will occaficn the greatelt exertions, 
when the mind is very unwilling'. There feems to be this 
difference between the mufcles obeying the will, and thofe 
which are governed by a vis infita; namely, that the lat¬ 
ter, being more irritable, are very ealily excited into mo¬ 
tion by a gentle ftimulus; as for inftance, the heart and 
inteftines ; which organs are moft manifeftly, and greatly, 
and conftantly, irritable. On the other hand, the mufcles, 
which obey the will, are neither endowed with fo great 
nor fo durable a power of this kind. Hence they either 
Hand in need of the power of the will, or of a ftronger fti¬ 
mulus; when they are excited by a ftimulus, they are ani¬ 
mated to motion againft the will. Thus it happens, that 
in apoplexies the mufcles which obey the will languifti, 
and become paralytic, as being deftitute of all influx from 
the brain ; while the vital mufcles, having no occafion 
for the operation of the brain, continue to be excited in¬ 
to contraction by their ftimuli, independent of the will, 
as the heart by the blood, and the inteftines by the air 
and aliments. 
The ftrength of the mufcles is very confiderable in all 
perfons., but more efpecially phrenetic and robuft men ; 
lince frequently, with the ufe of a few mufcles only, they 
will raife a weight equal to, or much greater than, that of 
the whole human body. In a healthy man, very (lender 
mufcles fuffice to lift two or three hundred pounds : the 
mufcles of the back will even fuftain 3000. Notwithftand- 
ing this, we fee, that much the greater part of the force 
or power exerted by a mufcle is always loft without pro¬ 
ducing any vifible effeft. For all the mufcles are inferted 
nearer the point or centre of motion, than the weights they 
are applied to; and therefore their aftion is weaker, in the 
fame proportion as they move a (liorter part of the lever 
than that to which the weight is applied. Moreover, in 
tnoft of the bones, efpecially thofe of the limbs, the muf¬ 
cles are inferted at very acute angles ; whence again the 
■effeCt which a mufcle exerts in aftion is proportionally le(s r 
as the fine of the angle intercepted between the bone and 
the mufcle is lefs than the whole fine. Again, the half of 
all mufcular force is loft; becaufe a mufcle, like an extend¬ 
ed cord, exerts as much force at its fixed as at its moving 
extremity. Befides, many of the mufcles are feated in the 
angle of two bones» and, when the joint is bent, the mufcle 
becomes alfo bent; confequently a quantity of force muft 
be firft applied to ftretch the mufcle, Jrefore the mufcle 
can aft on the bone in which it is inferted. Many of them 
pafs over feveral joints, each of w hich they bend in fome 
degree, whereby a lefs part of their remaining force goes 
to bend the joint to which they are particularly deftined. 
The fiefhy fibres themfelves of the mufcles frequently in¬ 
tercept angles with the tendon in which they terminate ; 
whence their force is diminifhed in proportion to the fine 
of the angle of their infertion. Finally, the mufcles move 
their oppofed weights with the greateft velocity and expe¬ 
dition, fo as not only to overcome the equilibrium, but 
likewife to add a confiderable celerity to the weight. 
All thefe Ioffes of power being computed, make it evi¬ 
dent, that the force exerted by mufcles in their contraftion, 
is exceeding great, and almoft beyond any mechanical com¬ 
putation. The effeft is fcarcely one-fixtieth of the whole 
force exerted by the mufcle, and yet only a fmall number 
of thefe mufcles, weighing but a few pounds, are able not 
only to raife fome thoufands of pounds, but alfo with a 
confiderable celerity. Nor is this to be reputed any defeft 
of wifdom in the Creator ; for all thole Ioffes of power 
were fieceffary for a juft fymmetry or proportion of the 
parts, and for various motions and celerities in different 
direftions. But we may, however, conclude from hence, 
ahat the aftion of the nervous or animal fluid is very power- 
1 O M Y. 
ful, fince, in an engine fo fmall, it can exert a force equal 
to fome thoufand pounds for a confiderable time, even for 
many days together; nor docs this feem to be otherwife ex¬ 
plicable, than by the incredible celerity by which the influx 
of this fluid obeys the command of the will. But how or 
whence it acquires fuch a velocity, is not in our power to 
fay; it is fufficient that w e know the laws of its motion are 
fuch, that a given action of the will produces a new and 
determinate celerity in the nervous fluid. 
There are various means by which the motions of the 
mufcles are rendered more fafe, certain, and eafy. The 
large long mufcles, by which the greater motions of flexure 
are performed, being included in tendinous capfules or ca¬ 
fes, drawn and tightened by other mufcles, are thus fecur- 
ed and ftrengthened; and thus the mufcle, in a ftate of con¬ 
traction, remains prefled againft the bone all the time that 
the limb is bent, and confequently avoids a confiderable lofts 
of its power. The long tendons, which are incurvated or 
extended over joints in their motion, are received and con¬ 
fined by peculiar ligaments, which retain them within they 
flippery channels ; and in thefe fheaths a particular liquor 
is feparated for the lubricating of the tendons. Nature 
makes a ftmilar provifion for thofe mufcles which per¬ 
forate others in their cotirfe. In other parts, the tendons 
are either carried round certain eminences of the bone, in 
order that they may be inferted at greater angles into the 
bone which they move ; or elfe they arc inferted into ano¬ 
ther bone, from whence a different tendon defeends under 
a larger angle into the bone to be moved, la other parts, 
the mufcles which are derived from convenient fituations, 
have their tendons carried round in a contrary direction 
by nature, fo that they pafs into the part to be moved as 
it were round a pulley. Nature has likewife furrounded 
the mufcles on all Tides with a confiderable quantity of fat, 
which is fpread alfo between the bundles of fibres, and 
alfo between the fmall fibres themfelves; which fat, being 
prefled out by the turgefcence of the mufcles and fibres, 
renders them foft, flexible, flippery, and fit for motion. 
Moreover, the power and aftion of one mufcle is deter¬ 
mined by the co-operations or oppofitions of others, which 
ferve either to hold firm fome part from whence the mufcle 
arifes, or to bend it together with the mufcle, or elfe to 
change its aftion from the perpendicular to the diagonal, 
by concurring to aflift its force at the fame time. Remote 
mufcles often aflift each other, the fuperior mufcles keep¬ 
ing the bone, whence the inferior acting mufcles arile 
firm and fteady. Therefore, the aftion of no one mufcle 
can be underftood from confidering it alone; but all thq 
others, which are either inferted into the mufcle itfelf, or 
into any of the parts to which the mufcle adheres, muft 
likewife be brought into the confideration. 
By thefe mufcles, varioufly confpiring and oppofingeach 
other, are performed walking, ftanding, flexion, extenfion, 
deglutition, and all the other geftures and offices of the 
feveral parts in the living body. But the mufcles have 
likewife fome common actions, by which they are of ufe 
to the whole animal. They haften the return of the venous 
blood, by prelfing it out both from the veins of the muf¬ 
cles themfelves, and from the veins which lie between 
them; for the blood in thefe veflels, diftributed between 
the turgid bundles of a contracted mufcle, is, by the valves, 
determined towards the heart: they likewife return the fat 
to the blood ; and ftiake, grind, or condenfe, the arterial 
blood, and return it quicker to the lpngs ; they contribute 
in a great degree to the fecretions and excretions. Again, 
in the liver, mefentery, womb, &c. they promote the 
courfe of the contained blood, bile, and other juices, fo 
as to leffen the danger of their ftagnation ; they ferve alfo 
to increafe the ftrength of the ftomach, by adding their 
own ftrength to it, whereby chgeftion is promoted ; info- 
much that all fedentary and inaftive courfes of life are 
contrary to nature, and pave the v/ay to difeafes from a 
ftagnation of the humours, or from a corruption or crudity 
of the aliments. The large mufcles, which are generally 
placed round any of the cavities of the body, propel the 
z blood 
