4 i6 DIGESTION 



alimentary canal much in the same way as the fats. Lecithin is 

 decomposed by pancreatic and intestinal juice into fatty acids and 

 glyceryl-phosphoric acid, and cholin is liberated. As regards the 

 behaviour of the sterins of the food little is known, but it is not 

 unlikely that their esters are split up, and the sterins thus set free 

 as well as those originally free in the food may then be absorbed, 

 in part at least, without further change. The starch and dextrin 

 which have escaped the action of the saliva are changed into 

 maltose by the amylase of the pancreatic juice, and the maltose 

 into dextrose by the maltase of the same secretion and of the succus 

 entericus. 



The succus entericus, in addition to its important functions 

 already mentioned, aids as an alkaline liquid in lessening the acidity 

 of the chyme and establishing the reaction favourable to intestinal 

 digestion. It will convert into monosaccharides any cane-sugar, 

 maltose, or lactose, which may reach the intestine; but it cannot 

 be doubted that some cane-sugar may be absorbed by the stomach, 

 after being inverted by the hydrochloric acid of the gastric juice 

 or by inverting ferments taken in with the food, or on its way 

 through the gastric walls. 



Upon the whole no great amount of water is absorbed in the small 

 intestine, or at least the loss is balanced by the gain, for the intestinal 

 contents are as concentrated in the duodenum as in the ileum. But 

 as soon as they pass beyond the ileo-caecal valve water is rapidly 

 absorbed, and the contents thicken into normal faeces, to which the 

 chief contribution of the large intestine is mucin, secreted by the 

 vast number of goblet cells in its Lieberkuhn's crypts. 



Bacterial Digestion. So far we have paid no special attention to 

 other than the soluble ferments of the digestive tract, although 

 we have incidentally mentioned the action of the lactic acid bacilli 

 on carbo-hydrates and of the fat-splitting bacteria on fats. It is 

 now necessary to recognize that the presence of bacteria is an 

 absolutely constant feature of digestion ; and although their action 

 must in part be looked upon as a necessary evil which the organism 

 has to endure, against the consequences of which it has to struggle, 

 and to which in all probability it has to a great extent adapted 

 itself, it is not unlikely that in part it may be ancillary to the pro- 

 cesses of aseptic digestion. But bacteria are not essential (in mam- 

 mals, at any rate, living on milk), as some have supposed. For it 

 has been shown that a young guinea-pig, taken by Caesarean section 

 from its mother's uterus with elaborate aseptic precautions', and 

 fed in an aseptic space on sterile milk, grew apparently as fast as one 

 of its sisters brought up in the orthodox microbic way. The ali- 

 mentary canal remained free from bacteria (Nuttall and Thierfelder). 

 On the other hand, chickens hatched from sterile eggs and kept in 

 a sterile enclosure lived, indeed, for a time, but did not thrive in 



