ANATOMY. 
562 
branous partitions and flripes of the cellular fubflance re¬ 
moving them farther from each other, till at laft a num¬ 
ber of them combined together, either parallel or inclined, 
are furrounded with a more thin and denfe cellular mem¬ 
brane, continuous with that of their partitions; and this 
being again furrounded by a thicker plate of the cellular 
fubflance, parts the whole bundle from the adjacent flefh, 
and gives it the denomination of a fngle or entire mvfcle. 
In every one of thefe threads there appears a fmaller feries 
of filaments, which, by oblique extremities, are cemented 
to others of the fame kind, forming together a large fibre. 
The generality of the muffle's, but more efpecially thofe 
which are inferted into the bones, have other fibres fixed 
to them; but thefe are condenfed into a more (lender, 
hard, and fhining, fubflance, of a filver colour, which has 
the name of tendon. When the tendinous fibres expand 
into a broad flat furface, they form what is called an apo- 
Jieurojis. The cellular texture which covers the whole 
tendon, is called its vagina or Jlieath, and refembles the 
coat of a mufcle. That fleihy fibres truly change into 
fuch as are tendinous, fays Dr. Haller, is evident from 
comparing a foetus, in which there are few tendons, with 
a child of fome years growth, in which there are many 
more; and both with an adult, in which there are the 
greatefi: number. But Dr. Wrilberg obferves, in oppofi- 
tion to this, that many tendons are found in a foetus, which 
could not afTume this nature by mufcular adion; as the 
tendo Achillis, aponeurofis plantaris, centrum diaphrag- 
matis, See. Befides, in various parts of the body there 
are tendons found without correfponding mufcles. 
Muffles which are not inferted into any of the bones 
have commonly no tendons, as the fphinbters and mufcu¬ 
lar membranes of the yifeera and vefiels. But thofe com¬ 
monly end in long tendons, which are required to pafs 
round the joints and heads of the bones, to be inferted in 
thofe extrenpties which are more moveable. ' In a foetus 
the mufcles are evidently inferted into the periofleum on¬ 
ly ; but, in adults, where the periofleum is more clofely 
joined with the bone itfelf, the tendons, being confufed 
•Vvith the periofleum, pafs together with that.even into the 
foveoli of the bone. The tendinous fibres, indeed, often 
lie in a ftraight line with the fleflty ones, and are as it were 
a continuation of them. But in many parts of the body 
the fleflty fibres are obliquely inclined to the tendon, and 
adhere to it ; and the tendon itfelf grows thicker in its 
progrefs by continually receiving new fibres. This is 
called a tendinous mufcle. Other tendons lie in the middle 
between two plates of fibres, which are inferted on each 
lide of the tendon at angles obtufe downward ; and this is 
called a pennated mufcle. There are inflances of numerous 
tendons, pennated in different places, formed into one muf¬ 
cle. There are alfo other methods by which the tendinous 
fibres are joined with the flefhy ones. 
Within the cellular tunic that furrounds the fibres, the 
arteries and veins, running in company with each other, 
are fub-divided into net-works. From the fmaller of thefe 
vefiels a vapour is exhaled into the thinner, and the fat is 
transfufed into the thicker, cellular fubflance; from 
whence again they are both abforbed by the lymphatic 
vefiels; which can be diflinCtly feen both on the furface 
and in the fubflance of the mufcles. The nerves of the 
mufcles are Hill more evident. They are commonly very 
large ; and enter by fo many branches, that fome anato- 
miffs have confidered the mufcular fibres as formed by 
them. Several arguments are offered againff this opinion : 
one of the firongeft is, that mufcles in the limbs of ani¬ 
mals do not fhrink, although the nerves entering thefe 
mufcles have been cut through for a confiderable length 
of time. The nerves enter the mufcles in a way fome- 
what fimilar with that of the arteries and veins; but it is 
impoflible to trace them a great way among the flefhy fi¬ 
bres, for they at length depofite their harder covering, and 
become fofr, and difappear before they can be traced to 
their terminations. 
The fabric of the leaft fibres, which are fuppofed to be 
the elementary particles of a mufcle, being invefligated by 
the miciofcope in man and other animals, is found to be 
fimilar to that of the larger fibres, and they are all joined 
together by the intermediate cellular fubflance. The fur¬ 
face of the fibres, however, puts on a curious waved or 
zigzag appearance. This undulated appearance Prof- 
chafka thinks is nothing elfe than impreflions made by the 
vefiels and cellular fubflance,' and perhaps by the nerves : 
but Dr. Monro has deferibed and delineated a fimilar ap¬ 
pearance in the tendons and nerves; and is of opinion, that 
they are to be confidered as folds or joints, ferving to ac¬ 
commodate the parts to the different flates of flexion and 
extenfion. In proof of this, he finds, that thofe parts 
which appear thus in their relaxed flate, lofe it when they 
are much flretched. 
With regard to the nature of the ultimate moving fibres, 
there have been many difputes. Some anatomifts think 
them folid; others, hollow, formed of a feries of vefiels 
or rhomboidul chains communicating with each other. 
Some, again, have been of opinion, that they are full of a 
kind of down or woolly fubflance, &c. It may be afked. 
Whether they are hollow ? w hether they are continued 
from the arteries ? or whether the difference between muf¬ 
cular and tendinous fibres, lies in the latter being render¬ 
ed more denfe, and beat clofer together by an expulfion 
of the fluids? That thefe are not probable, appears from 
the minutenefs of the fibres, which are found lefs than 
the red-blood globules, and from the whitenefs of a muf¬ 
cle after the blood is waffled out of it. The flruCture of 
the tendons and aponeurofes agrees in fome refpecls with, 
but differs in others from, that of the mufcles. We obq 
ferve their fibres regularly difpofed, and feparated by cel T 
lular fubflance and blood-veffels ; and without doubt they 
have lymphatics and nerves: but the tendinous fibres are 
clofer together than thofe of the mufcles, the cellular 
fubflance which feparates them is finer, their red vefiels 
are fewer in number, and the nerves cannot be traced, 
without difficulty, into their fubflance. It has been doubt¬ 
ed whether the fibres of tendons are a continuation of the 
moving fibres, or of a different nature. Many, both an¬ 
cients and moderns, have embraced the firft opinion, others 
the feconcf; but, if we confider that the tendinous fibres 
are not irritable, have no contraction, that they differ lit¬ 
tle from thofe which conflitute the ligaments, and that 
they degenerate fometimes into a fubflance truly cellular, 
we fhould be inclined to adopt the latter opinion. 
According to Dr. Haller, a mufcle is endowed at leaf! 
with a threefold force. Firft the vis mortua, in common 
to it with other animal fibres. Another, which he has 
called the vis infita, and which has different phenomena 
from the former. It is more peculiar to life ; and,, though 
it may continue for a few hours after death, yet it difap- 
pears much fooner than the former. Again, in molt cafes, 
it atls by alternate ofcillations; fo that, being driven hi¬ 
ther and thither, it fometimes contrails the- mufcle to¬ 
wards the middle; fometimes again it extends the mufcle 
from the middle towards the extremities,. and fometimes 
alfo it has a reiterated motion. Moreover, it is manifeftly 
quicker, and performs the greatefi motions. It is exciteq 
both by the pricking w r ith a fharp inflrument, and in the 
hollow mufcles by inflated air, by water, and eyery kind 
of acrimony, but moft powerfully of all by a torrent of 
eleCtrical matter. Laftly, it is peculiar to the mufcular 
fibre, and is found in'no other part of the human body wgh 
the qualities above-mentioned. But we muft give a more 
particular explication of its'phenomena. , 
It is natural to every mufcle to fhorten itfelf, by drawing 
the extremities towards its belly or middle. But to dis¬ 
cover the moving power of a mufcle from the fabric which 
we have defenbed, it will be of life to confider the appear¬ 
ances obfervable in the mufcular contraction. Every muf¬ 
cle then becomes fflorter and broader in its aCtion. But 
this contraction of its length is various ; in fome more, 
in others lefs ; and is very confiderable in fome of the 
fphinders, infomuch that they, appear to be contracted 
more 
