198 PHYSIOLOGY AT THE FAEM. 



animals, being very much greater in vegetable-feeders. In 

 carnivorous animals, like the dog, the jaw has but two mo- 

 tions namely, the hinge-like movement by which the mouth is 

 opened and shut ; the teeth of the one jaw cannot glide over 

 the teeth of the other. Thus, in the dog and animals of the 

 like character, mastication consists merely in the cutting, tear- 

 ing, and chopping of the aliment, to which operations the 

 incisor, the canine, and the molar teeth in the dog are sev- 

 erally adapted. 



Besides the mere hinge-motion of the jaw, common to her- 

 bivorous animals with the carnivorous, the herbivorous can 

 carry the jaw forwards, backwards, and to either side. There 

 is a peculiarity in the jaw of herbivorous animals besides 

 namely, that the under jaw is narrow from side to side as com- 

 pared with the upper jaw, the effect of which is that the molar 

 teeth of the lower jaw cannot be made to fit against the cor- 

 responding teeth of the upper jaw on the two sides at once. 

 When the teeth of the two jaws are brought into a line on the 

 one side, the teeth of the other side are no longer in a line, 

 whence it follows that in these animals mastication must be 

 unilateral ; and it is observed that in such animals the one 

 side of the mouth is used in mastication to the exclusion of 

 the other side for long periods, as for a quarter of an hour, 

 half an hour, or even an hour. 



Mastication is a very slow process in the horse and other 

 herbivorous animals that do not ruminate. In those that 

 ruminate, the first mastication is very rapid ; the second 

 mastication, however, after rumination, is a slow process. 



The following statement is made as the result of observa- 

 tions on the mastication of the horse. A horse of moderate 

 size took an hour and a quarter to masticate 4 pounds and 

 some ounces of dry hay, which he swallowed in from 60 to 65 

 portions. A large horse took an hour to the same quantity, 



