100 NATURAL THEOLOGY. 



problem is to pull the lower jaw cloivn. The obvious method 

 should seem to be, to place a straight muscle — namely, to 

 fix a string from the chin to the breast, the contraction ol 

 which would open the mouth, and produce the motion re- 

 quired 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 w^ant must be effectuated by some mus- 

 cular mechanism disposed further back in the jaw. The 

 mechanism adopted is as follows : 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 con- 

 verted in its progress into a round tendon. Now it is man- 

 ifest that the tendon, while 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 ascend ; and having thus 

 changed its line of direction, is inserted into the inner part 

 of the chin : by which device, namely, the turn at the loop, 

 the action of the muscle — which in all muscles is contrac- 

 tion — 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, namely, a slit in one tendon to let another tendon 

 pass through it ? This structure is found in the tendons 

 which move the toes and fingers. i The long tendon, as it 

 is called, in the foot, which bends the first joint of the toe, 

 passes through the short tendon which bends the second 



* See a similar contrivance in Plate II., Fig. 1. 



t Plate IV., Fig. 1. a, is the tendon of the long flem f th 

 iocs, which divides about the middle of the foot into fou" por'.ions, 

 which pass through the slits in 6, the short flexor tendons. 



