The BOOK of 
particularly their legs, and the mufcles of thofe 
parts, which fwell and extend every way ina 
furprifing manner, by means of the blood and 
humours driven into them, juft as if they had 
been injected with additional liquors; fo that 
in courfe of time, they are as it were unnatu- 
rally f{tretched, and bent in the manner of a 
bow. But this alteration obtains chiefly in in- 
fects, whofe mufcles alfo move much longer 
than thofe of any other {pecies of animals; and 
even retain their motion, in many {pecies, after 
the head has been feparated from the body. 
We obferve alfo, that as foon as infe&ts break 
from their old fkins, their bodies grow larger 
every way, almoft inftantaneoufly; and the 
fame thing happens, in proportion, to animals 
whofe blood has an extraordinary degree of 
heat in it. Hence it is, that their mufcles en- 
deavour the more violently to contract and draw 
themfelves together. In fine, we likewife very 
plainly fee that the mufcles, when firft they 
begin to exert their powers, grow much redder, 
on account of the blood penetrating at that 
time into their fubftance, and in fome degree 
expanding it; and they become likewife much 
larger, by means of the blood-veffels which 
run through them, and extend their moving 
fibres. 
From thefe particulars it evidently follows, 
that every contraction of a mufcle mutt be pre- 
ceded by fome degree of dilatation ; and this 
dilation I fuppofe to be of three kinds. The 
firft is performed in the natural and voluntary 
contractions of the mufcles, by the blood forced 
into, and thereby partly dilating them. The 
fecond, which obtains, in natural contractions, I 
attribute to the contents, which byextending and 
dilating the moving fibres, and thereby drawing 
the blood more copioufly to them, occafions a 
relaxation of their contractile powers. ‘The 
third kind of dilation is that which precedes 
the voluntary contractions, and feems to be 
produced by the determination of the antago- 
nift mufeles; for thefe act upon the mutfcles, 
oppofed to them, in the fame manner that 
the contents act upon mufcles, whofe motions 
are natural, 
- Now what does that fubtile matter, which 
conitantly flows through the nerves into the 
mufcles, contribute to their contraction? Is it 
derived to the moving fibres, ferving to open 
130 
fome of the blood-veffels that furround the- 
nerves lying within the mufcle? Or does it, by 
mixing with the blood, make it fuddenly effer- 
vefce and ferment, and fo excite that motion, 
by which the mufcles may again fhake off fuch 
fubtile matter, fo as to caufe an immediate 
contraGtion of the moving fibres? I muft own 
myfelf. unable to give a fatisfactory anfwer to 
any of thefe queftions, and therefore leave 
them, as a moft fruitful matter of contempla- 
tion, to the difcuffion of others. . 
As to the other things which I have hitherto 
propofed, I think I may thence moftt fairly con- 
clade: Firft,; Phat all the mufcles are naturally 
contracted ; that is, they are in a ftate of con- 
traction before they have performed any mo- 
Nee 4 Us RIE. ver, 
tion. Secondly, That their contraction is, in 
part, deftroyed by the blood, and fuch other ~ 
humours as flow into them from’ their particu- 
lar veflels; and that this is, as it were, the firft 
caufe of the dilatation or expanfion of the muf- 
cles, though they {till perfift in a ftate of con- 
traction in a lefler degree. By this power alfo, 
the circumambient air is driven out of its place, 
and becomes condenfed, in the fame propor- 
tion wherein the mufcles are expanded. Third- 
ly, The contents of the vifcera, cavities, and 
little tubes of the body, about which the 
moving fibres are difpofed, contribute greatly, 
as a fecundary caufe, which cbtains in natural 
motions, to the complete diftention or dilata- 
tion of the mufcles; whereas, in voluntary 
motions, fuch complete diftention and dilata- 
tion muft be attributed chiefly to the contrary 
determination of the antagonift mufcles: for 
the moving fibres in thefe two fpecies of muf- 
cles, which are differently fituated, are confi- 
derably expanded by thefe two caufes, which 
likewife difpofe the blood-veffels, belonging to 
the mufcles, to receive a far greater quantity of 
blood; fo that the dilatation being once arrived 
at its higheft pitch, the returning contraction 
may be the more powerful. Fourthly, The 
air, already repelled and condenfed, contributes 
greatly to produce this effect; for, by being 
immediately urged to dilate itfelf by the equi- 
librium of the atmofphere, it, in its turn, aéts 
with the greater violence upon the mufcles, to 
oblige them to return into their late ftate of 
contraction; which is befides fo natural to 
them, that they would reaffume it of them- 
felves, fooner or later, without any fuch com- 
pulfion. Fifthly, We muft add to the caufes 
already afligned, the perpetual and natural ir- 
ritations which the nerves are conftantly excit- 
ing in the moving fibres of the mufcles them- 
felves, and which are continually engaged in 
urging them to contraction. I would here be 
underftood to mean thofe ftimuli, which are 
produced by the blood in its circulation, and in 
its paflage through the arteries to the origin of 
the fpinal marrow, and all the nerves; or elfe 
thofe ftimuli which are communicated to the | 
beginning of the {pinal marrow, and the nerves, 
by external obje&ts, which make an impreffion _ 
upon the blood. Hence therefore, in the fixth | 
place, the mufcles, as well the natural and 
moveable ones as thofe fubje&t to the diétates 
of the will, are neceffarily difpofed, and in a 
manner con{trained, to return to their original 
and natural ftate of contraétion. Hence, fe- 
venthly, I may reafonably conclude, that in 
all the reciprocal contractions of the mufcles, | 
their contents are again forcibly difcharged, 
fince the moving fibres, which were before 
extended, at this time come nearer again to 
each other, and become very compact, fo as 
to recover the fituation they were in, previous 
to their being dilated. For this reafon alfo, 
they muft then take up lefs room, though we 
may fee fume tumours arife at this time on the 
furface of the mufcles; thefe turnours being 
produced by the extraordinary compactnefs of 
the 
