SEC. 8. THE CHANGES WHICH THE FOOD UNDERGOES 

 IN THE ALIMENTARY CANAL. 



278. Having studied the properties of the digestive juices 

 as exhibited outside the body, and the various mechanisms by 

 means of which the food introduced into the body is brought 

 under the influence of those juices, 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 contents of the canal are gradually converted into 

 faeces. The events which lead to this conversion are two-fold. On 

 the one hand the digestive juices do bring about, inside the alimen- 

 tary canal, changes which in the main are the same as those observed 

 in laboratory experiments outside the body and described in previous 

 sections, though the results are somewhat modified by the special 

 conditions which obtain within the body. On the other hand 

 absorption, that is to say, the passage from the interior of the canal 

 into the blood vessels and lymphatics, of digested material in com- 

 pany with water, is going on along the whole length of the canal, 

 and especially in the small and large intestines. It will be con- 

 venient to confine ourselves at present to the study of the first 

 class of events, the changes effected in the canal, merely noting 

 the disappearance of this or that product, and deferring the 

 difficult problem of how absorption takes place to a subsequent and 

 separate discussion. 



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 



