OF THE MUSCLES. 
85 
general, the vivacity of their motions would be ill exchanged 
for greater force under a clumsier structure. 
We have offered our observations upon the structure of 
muscles in general; we have also noticed certain species 
of muscles; but there are also single muscles, which bear 
marks of mechanical contrivance, appropriate as well as 
particular. Out of many instances of this kind, we select 
the following:— 
I. Of muscular actions, even of those which are well 
understood, some of the most curious are incapable of pop¬ 
ular explanation; at least, without the aid of plates qnd 
figures.* This is in a great measure the case, with a very 
familiar, but, at the same time, a very complicated motion— 
that of the lower jaw; and with the muscular structure by 
which it is produced. One of the muscles concerned may, 
however, be described in such a manner, as to be, I think, 
sufficiently comprehended for our present purpose. The 
problem is to pull the lower jaw down. The obvious method 
should seem to be, to place a straight muscle, viz. to fix a 
string from the chin to the breast, the contraction of which 
would open the mouth, and produce the motion required at 
once. But it is evident that the form and liberty of the 
neck forbid a muscle being laid in such a position; and 
that, consistently with the preservation of this form, the 
motion, which we want, must be effectuated by some muscu¬ 
lar mechanism disposed farther back in the jaw. The me¬ 
chanism adopted is as follows: [PI. XV. fig. 1, 2.] A 
certain muscle called the digastric, rises on the side of the 
face, considerably above the insertion of the lower jaw, 
and comes down, being converted in its progress into a 
round tendon. Now, it is evident, that the tendon, whilst 
it pursues a direction descending towards the jaw, must, 
by its contraction, pull the jaw up, instead of down. What 
then was to be done? This, we find is done: The de¬ 
scending tendon, when it is got low enough, is passed 
through a loop, or ring, or pulley, in the os hyoides, and 
then made to ascend: and, having thus changed its line of 
direction, is inserted into the inner part of the chin: by 
which device, viz. the turn at the loop, the action of the 
muscle (which in all muscles is contraction) that before 
would have pulled the jaw up, now as necessarily draws it 
down. “ The mouth,” says Heister, “ is opened by means 
of this trochlea in a most wonderful and elegant manner.” 
II. What contrivance can be more mechanical than 
* The want of the aid of plates and figures, which the author here ex¬ 
presses, is now supplied in this Boston edition. 
H 
