DIGESTIVE ORGANS. 



515 



Fig. 212. 



the sterno-thyroideus, and the omo-hyoideus; the names of which indi- 

 cate their origin and insertion. These, in the aggregate, form a mus- 

 cular chain, which, when it makes the trunk its fixed point, depresses 

 the lower jaw. The arrangement of the elevators and depressors is 

 such, that the former predominate over the latter; and hence during 

 sleep the jaws continue applied to each other, and the mouth is conse- 

 quently closed. 



The human organs of mastication hold an intermediate place between 

 those of the carnivorous and herbivorous animal. In the carnivorous 

 animal, which has to seize hold of, and retain its prey between its teeth, 

 the jaws have considerable strength; and the movement of elevation is 

 all that is practicable ; or, at least, that can be effected to any extent. 

 This is dependent upon organization. The condyle is broader from side 

 to side, which prevents motion in that direction: the glenoid cavity is very 

 deep, so that the head of the jaw-bone cannot pass out of it; and it 

 is, moreover, fixed in its place by two eminences before and behind. 

 The muscular apparatus is also so arranged as to admit of energetic 

 action on the part of the muscles that raise the jaw; but of scarcely 

 any in a horizontal direction. The deep impressions in the regions of 

 the temporal and masseter muscles indicate the large size of these mus- 

 cles in the purely carnivorous animal ; whilst the pterygoid muscles are 

 extremely small. The teeth, too, are characteristic ; the molars being 

 comparatively small, at the same time that they are much more pointed. 

 On the other hand, the cuspidati are 

 remarkably large, and the incisors, in 

 general, acuminated. 



The herbivorous animal has an ar- 

 rangement the reverse of this. The 

 condyle or head of the lower jaw is 

 rounded; and can, therefore, be moved 

 in all directions; and as easily hori- 

 zontally as up and down. The glenoid 

 cavity is shallow, and yields the same 

 facilities. The articula-tion, which is 

 very close in the carnivorous animal, is 

 here quite loose. The levator muscles 

 are much more feeble; the temporal 

 fossa is less deep; the zygomatic arch 

 less convex; and the zygomatic fossa 

 less extensive. On the other hand, 

 the pterygoid fossa is ample and the 

 muscles of the same name are largely 

 developed. The molares are large and 

 broad; and their magnitude is so great 

 as to require, that the jaw should be 

 much elongated in order to make room 

 for them. 



The joint of the lower jaw has, in man, solidity enough for the jaws 

 to exert considerable pressure with impunity, and laxity enough that the 

 lower jaw may execute horizontal movements. The action of the leva- 



Skull of the Cow. 



