CHAP. XVI.] ALIMENTATION. 197 



The mechanical subdivision, bruising, and crushing of the food, 

 accomplished by the teeth and the muscular contractions of the 

 walls of the alimentary canal, is merely a process of preparation 

 for the solvent action of the digestive juices. Of these juices 

 there are five, each having^a special action. 



(1) The saliva, containing the digestive enzyme ptyalin, 

 transforms starch into sugar. 



(2) The gastric juice, containing the enzyme rennin, and 

 pepsin (an enzyme acting in the presence of an acid), trans- 

 forms proteids into peptones. 



(3) The pancreatic juice, containing trypsin (an enzyme 

 acting in the presence of an alkali), transforms proteids into 

 peptones, and, by virtue of other constituents, transforms 

 starch into sugar, and emulsifies fats or turns them into solu- 

 ble soaps. 



(4) Bile, containing cholesterin, bile-salts, and other matters, 

 assists the pancreatic juice in saponification and emulsion of 

 fats, promotes absorption of the same, and modifies putrefactive 

 changes in the intestine. 



(5) Intestinal juice, containing mucus, transforms all food- 

 stuffs in a feeble fashion not clearly demonstrated nor under- 

 stood. 



All material that these solvents fail to transform into a soluble 

 and absorbable condition is gradually worked downwards by 

 the peristaltic contractions of the alimentary canal, and finally 

 leaves the body as waste and useless matter. 



NOTE. For the sake of simplicity, we have considered digestion in a broad 

 way as the conversion of practically non-diffusible proteids and starch into more 

 diffusible peptones and highly diffusible sugar, and as the emulsifying and split- 

 ting up of fats. There is reason to believe that some of the sugar may be 

 changed into lactic acid, or even into butyric or other acids, and that some of 

 the proteids are carried beyond the peptone condition. But there is no doubt 

 that the greater part of the proteid is absorbed as peptone, that carbohydrates 

 are mainly absorbed as sugar, and that the greater part of the fat passes into 

 the body as an emulsion. 



Absorption. We have now to consider how the products of 

 digestion find their way out of the alimentary canal into the 

 tissues of the body ; for, properly speaking, though the food 

 may be digested and ready for nutritive purposes, it is, until 

 it passes through the walls of the alimentary canal, still practi- 

 cally outside the body. 



