672 MAMMALIA. 



to bruise, to cut, to tear, or to grind alimentary substances, we 

 must naturally expect the movements of which the lower jaw is 

 capable, to be in correspondence with the nature of the dental 

 apparatus. 



In MAN, as the student well knows, in consequence of the 

 laxity of the ligaments that connect the inferior maxilla with the 

 temporal bone, and the thickness of the articular cartilage that 

 is interposed between the convex surface of the condyle and the 

 shallow glenoid cavity, every kind of motion is permitted in con- 

 formity with the omnivorous habits of the human race; and the 

 temporo-maxillary articulation is no longer a mere hinge, but 

 the teeth can be made to act upon each other by rubbing their 

 grinding surfaces in all needful directions. In the Herbivorous 

 quadrupeds these triturating motions are likewise extensive. In 

 the RODENTIA the movements of the lower jaw are principally 

 backwards and forwards, thus giving free play to their chisel-like 

 teeth whilst employed in eroding hard substances ; and in the 

 CARNTVORA, where there is no necessity for any grinding motion, 

 the condyle is so locked into a deep and transverse glenoid cavity, 

 that the movements of a hinge only are permitted. 



(750.) But, whatever the degree of motion conferred upon the 

 lower jaw, the muscles that act upon it are exactly comparable to 

 those of the human subject. The masseter is strengthened in 

 proportion to the hardness of the substances used for food ; the 

 temporal covers a greater or less extent of the cranium, as the 

 jaws are stronger or more feeble; and even the pterygoid muscles 

 differ only in relative size and form from those of Man. 



The digastric muscle, however, which is an important agent 

 in depressing the lower maxilla, does not preserve the same ar- 

 rangement in the lower quadrupeds that it presents in the human 

 species. In Monkeys indeed it still exhibits two fleshy bellies, 

 and a central tendon that traverses the stylo-hyoideus ; but in 

 general it is a single fleshy muscle, arising from the neighbourhood 

 of the mastoid process, and inserted near the angle of the jaw. 



(751.) The tongue in nearly all the Mammifera is composed 

 of the same muscles as in Man ; and their disposition is so similar, 

 as to render any detailed enumeration of them quite unnecessary. 

 The only exceptions worthy of notice are found in the Ant-eaters 

 (Myrmecophaga), and in the Echidna, animals possessing tongues 

 of remarkable length and slenderness, by means of which they 

 secure their insect prey. 



