152 NATURAL THEOLOGY. 



contraction of which would open the mouth, and 

 produce the motion required at once. But it is 

 evident that the form and hberty of the neck for- 

 bid a muscle being laid in such a position ; and 

 that, consistently with the preservation of this 

 form, the motion which we want must be effectu- 

 ated by some muscular mechanism disposed fur- 

 ther back in the jaw. The mechanism adopted is 

 as follows: — A certain muscle, called the digas- 

 tric, 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 manifest 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 descending tendon, when it is 

 got low enough, is passed through a loop, or ring, 

 or pully, in the os hyoides, and then made to as- 

 cend ; and having thus changed its line of direc- 

 tion, 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 con- 

 traction,) 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 following, viz., a slit in one tendon to let 

 another tendon pass through it? This structure 

 is found in the tendons which move the toes and 



