MASTICATION. 243 



anterior extremities, having the long axis of the mouth in a line with 

 the oesophagus. Thus, among certain rodents and arboreal insectivora, 

 which habitually sit erect while feeding, holding their food between their 

 fore feet, the anterior bellies of the digastrics are large and united, and 

 the intermediate tendons well developed and connected by faspial bands 

 with the hyoid bone, and by their deep surfaces with the mylo-hyoid 

 muscles, as in the rat and the common dormouse. In the water-vole 

 (Arvicola amphibius), however, the digastrics are connected together in 

 front by fascia alone, and the upper margin only of their middle part 

 is tendinous, and not connected with the hyoid bone. These animals 

 live on vegetable substances obtained while swimming, and habitually 

 hold the head stretched out in a line with the bocby. 



The mouth is closed by elevation of the lower jaw, and is the 

 reversal of the previous motion. Here powerful muscular action is re- 

 quired, since in the closure of the jaws, in many cases, great force is 

 needed. It is accomplished by the temporals, the masseter, and pter}'- 

 goicl muscles. In carnivora the temporal is the principal elevator of the 

 lower jaw. Its volume is proportionately enormous, the temporal fossae 

 occupying the entire surface of the parietal bones back to the occipital 

 spine. In herbivora and rodents the masseter muscles are the most 

 highly developed; their origin being from the zygomatic arch and a 

 portion of the superior maxillary bone, and being inserted in the lower 

 jaw, in the solipedes on both faces of the coronoid processes, as far 

 back as the last molars. Both of these muscles act as levers of the 

 third class, as is very evident in the rabbit, where the coronoid processes 

 are much lower than the articulation of the lower and upper jaw. In 

 the carnivora, the coronoid processes being separated by a considerable 

 space from the condyle, the conditions are most favorable for the action 

 of the temporal muscle. From the oblique direction of its fibres it tends 

 to produce drawing back of the lower jaw, where, as in the herbivora, 

 this is possible. 



The masseter muscle is developed in inverse proportion to the tem- 

 poral. It is, therefore, the principal elevator of the jaw in the herbivora 

 and in the rodents. It rises from the zygomatic spine in solipedes, the 

 maxillary tubercle in ruminants, to be inserted in the lower jaw. It is 

 also a lever of the third class ; its fibres are directed backward and 

 downward in the herbivora, and it may serve, therefore, as in the rodents, 

 to assist in the forward motion of the lower jaw ; its greatest power is de- 

 veloped when the resistance to the elevation of the lower jaw is between 

 the molar teeth, while its origin, being on a plane external to its insertion 

 in the lower jaw, as in the horse and rabbit, it may aid in lateral motion 

 of the jaw in animals where this motion is not rendered impossible by 

 the mode of articulation of the jaws, or by the overlapping of the teeth. 



