MASTICATION. 



267 



in the horse for the same amount of food, while the second mastication 

 is proportionately lengthened. As the teeth become worn away, masti- 

 cation becomes more and more difficult, and proportionately more and 

 more prolonged. In the horse the molar teeth are used up faster than 

 the incisors, and if it were not for the fact that the incisors become more 

 and more horizontal, the molar teeth could no longer come in appo- 

 sition. The influence of the secretion of saliva on mastication has been 

 determined by Colin experimentally, by making a fistula of the duct of 

 the parotid glands and allowing the saliva to escape externally from the 

 mouth. His results are shown in the following table : 



As regards the importance of thorough mastication, it is hardly 

 necessary to add anything further. We have found that its importance, 

 of course, varies in accordance with the nature of the food. Carnivora, 

 as has been mentioned, require mastication simpty to be perfect enough 

 to tear their food into pieces small enough to be swallowed, and in the 

 herbivora we reach the opposite extreme, and find there the group of 

 animals in whom a thorough mastication is of the utmost necessit} 7 . 

 From our considerations of the nature of vegetable foods w^e know that 

 the nutritive principles of these foods are contained within resisting, 

 tenacious envelopes. To enable these substances to be acted upon by 

 the digestive juices, and therefore to be absorbed, these envelopes must 

 be first mechanically ruptured, and this in the herbivora is the main 

 object of mastication. 



Where we find mastication imperfectly performed, we have, as an 

 invariable sequence, imperfect digestion, and we find that the grasses and 

 seeds, and so on, which escape mastication pass through the intestinal 

 canal entirely unaltered and are found in the excreta unchanged, and, in 

 the case of seeds, without even having lost their power of germination. 



