422 DIGESTION 



contents are as concentrated in the doudenum 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 Lieberkiihn'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 

 comparison with the control animals, and died at latest after eighteen 

 days (Schottelius) . It is probable that the difference in the results 

 is to be attributed to the difference in the food, purely vegetable 

 food requiring the aid of bacteria for its proper digestion, especially 

 for the decomposition of the cellulose, while an easily-digestible 

 food like milk does not. 



Among the more important actions of bacteria on the protein 

 food-products in the intestines may be mentioned the formation 

 of indol, phenol, and skatol, the first having tyrosin for its precursor, 

 and being itself after absorption the precursor of the indican in the 

 urine; the second being to a small extent thrown out with the faeces, 

 but chiefly absorbed and eliminated by the kidneys as an aromatic 

 compound of sulphuric acid; the third passing out mainly in the 

 faeces. 



The view put forward by Metchmkoff, that in the putrefactive 

 bacteria of the intestine the body carries within itself the seeds of 

 premature decay, owing to the harmful effects of absorbed products 

 of decomposed protein, cannot be looked upon as established, 

 although certainly the prophylaxis suggested by him (the increase 

 of the lactic acid content of the intestine by the addition of sour 

 milk, butter-milk, etc., to the diet) might well be a useful modifica- 

 tion of the dietetic habits of many persons, especially if associated 



