214 DIGESTIVE SYSTEM. 



quantities : according to Kemmerich the exclusive use of the 

 extract of meat wouM kill sooner than starvation. 



In studying the different phases of the act of digestion, we 

 will take, first, those which are observed in the sub-diaphrag- 

 matic part of the canal ; next, those of the cavity of the 

 stomach ; and, finally, phenomena which take place in the pas- 

 sage through the intestinal tube (large and small intestine). 



II. FIRST PART OF THE ACT OF DIGESTION. 



THE aliments introduced into the cavity of the mouth are 

 divided by the teeth (mastication), moistened and modified 

 by the saliva (salivation), and then carried into the pharynx, 

 seized by it, and pushed into the stomach by the oesophagus 

 (deglutition). 



A. Mastication. 



The purpose of mastication is to divide the solid aliments 

 so that they may be more easily attacked by the digestive 

 fluids of the mouth and other parts of the intestinal canal. 

 Meat and nitrogenous substances are more easily digested in 

 the stomach after they have undergone mastication in the 

 mouth, but the operation need not be carried very far in the 

 case of aliments of this kind : thus we observe that the exclu- 

 sively carnivorous animals have no teeth properly so called, 

 but merely hooks, with which they tear their food into large 

 pieces. Mastication is indispensable, on the contrary, in the 

 case of aliments belonging to the vegetable kingdom ; the 

 greater number of nutritive vegetable matters are enclosed 

 in a casing which generally resists the action of the digestive 

 juices: the masticating system serves to tear the cells, the 

 envelope of seeds, etc. ; prima digestio fit in ore, said the 

 ancients : in saying this, they spoke only of mastication, 

 being ignorant of the chemical process which takes place 

 during salivation. 



The lower jaw, as it rises and falls, represents a lever, 

 moving round a supposed axis, which, in movements of slight 

 extent, is centred in the condyles; but when the mouth 

 is wide open, the separation of the jaws is greater, and the 

 condyles quit the glenoid cavities, and come further for- 

 ward. The movement then takes place round an axis cross- 

 ing the two upright branches of the inferior maxillary at the 

 level of the dental foramen ; however little the buccal cavity 

 may be opened, and even in ordinary mastication, the two 



