306 THE TISSUES AND MECHANISMS OF DIGESTION. 



nicotin, appear to act directly on the walls of the canal, others, such as 

 strychnia, produce their effect chiefly by acting through the central nervous 

 system. 



THE CHANGES WHICH THE FOOD UNDERGOES IN THE ALIMENTARY 



CANAL. 



239. 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 feces. 

 The events which lead to this conversion are twofold. 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 out- 

 side 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 bloodvessels 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 con- 

 fine 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 prod- 

 uct, 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 movements 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 insufficient to produce any marked con- 

 version of the starch it may contain. During the rapid transit through the 

 oesophagus no appreciable change takes place. 



The amount of absorption of digested material, or even of simple water, 

 from the mouth or oesophagus must always be insignificant. 



TJie Changes in the Stomach. 



240. The arrival of the food, the reaction of which is either naturally 

 alkaline, or is made alkaline, or at least is reduced in acidity, by the addi- 

 tion of saliva, causes a flow of gastric juice. This, already commencing 

 while the food is yet in the mouth, increases as the food accumulates in the 

 stomach, and as, by the churning gastric movements, one part after another 

 of the food is brought into contact with the mucous membrane. 



The characters of the juice appear to change somewhat as the act of 

 digestion proceeds. The amount of pepsin in the gastric contents increases 

 for some time after food is taken, and probably the actual secretion increases 

 also. The acidity of the gastric contents is at first very feeble ; indeed in 

 man, in some cases at least, for some little time after the beginning of a meal 

 no free acid is present, and during this period the conversion of starch into 



