SEC. 6. THE CHANGES WHICH THE FOOD UNDER- 

 GOES IN THE ALIMENTARY CANAL. 



228. 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 fseces. The events which lead to this conversion are two- 

 fold. On the one hand the digestive juices do bring about, 

 inside the alimentary 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 company with water, is going 

 on along the whole length of the canal, and especially in the 

 small and large intestines. It will be convenient 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 conve- 

 nient 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 



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