MASTICATION. 



241 



forward and backward motions are possible, but are not present in as 

 great a degree in this case as in the two preceding types of herbivora. 

 These animals, therefore, occupy a mean between the rodents and 

 ruminants (Fig. 92). 



1. The Movements or the Jaws. — The mouth is opened by depres- 

 sion of the lower jaw, which is 

 effected in all animals by the 

 digastric muscles, aided, in the 

 horse, by the stylo-maxillary 

 muscle, which is in . reality a 

 short branch of the former. The 

 lower jaw is depressed very 

 largely by gravity ; hence, in all 

 animals we lind such a slight 

 muscular power acting as de- 

 pressor of the lower jaw as con- 

 trasted with the large number 

 of powerful muscles which pro- 

 duce its elevation. When the 

 mouth is opened the maxillary 

 condyle turns on its axis and 

 its posterior part, which, when 

 the jaws are closed, is in con- 

 tact, as in the horse, with the 

 subcondyloid apophysis, leaves this surface and moves anteriorly. 

 In carnivorous animals the condyle being fixed in a gutter-like glenoid 

 cavity, rotation on its axis is the only motion which is noticed. 



Fig. 90.— Head op Horned Ruminant— Ox. 

 (Biclard.) 



Fig. 91.— Head of Hornless Ruminant— Camel. (Biclard.) 



In all animals the finger placed below the zygomatic fossa will 

 distinguish the forward and backward motion of the coronary process 

 as the mouth is opened and closed. In carnivora the extent to which 



16 



