SEC. 4. THE CHANGES WHICH THE FOOD UNDERGOES 

 IN THE ALIMENTARY CANAL. 



Having studied the properties of the digestive juices, and the 

 various mechanisms by means of which the food is brought under 

 their influence, we have now to consider what, as matters of fact, 

 are the actual changes which the food does undergo in passing 

 along the alimentary canal, what are the steps by which the 

 food is converted into faeces. 



In the mouth the presence of the food, assisted by the move- 

 ments of the jaw, causes, as we have seen, a flow of saliva. By 

 mastication, and by the addition of mucous saliva, the food is 

 broken into small pieces, moistened, and gathered into a convenient 

 bolus for deglutition. In man some of the starch is, even during 

 the short stay of the food in the mouth, converted into sugar ; for 

 if boiled starch free from sugar be even momentarily held in the 

 mouth, and then ejected into water (kept boiling to destroy the 

 ferment), it will be found to contain a decided amount of sugar. 

 In many animals no such change takes place. The viscid saliva of 

 the dog serves almost solely to assist in deglutition ; and even the 

 longer stay which food makes in the mouth of the horse is in- 

 sufficient to produce any marked conversion of the starch it may 

 contain. During the rapid transit through the (BSOphagTlS no 

 appreciable change takes place. 



In the stomach, the arrival of the food, the reaction of which 

 is either naturally alkaline, or is made alkaline, or at least is 



