SURVEY OF DIGESTION AS A WHOLE 417 



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 Metchnikoff, 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 

 with a reduction in the total amount of protein consumed. That 

 the intestinal contents may include substances capable of inducing 

 severe toxic symptoms if absorbed unchanged scarcely needs proof. 

 Filtered extracts of faeces from normal persons made with salt 

 solution cause, when injected in small amounts into the circulation 

 of dogs, a fall of blood-pressure which may be speedily recovered 

 from or may be quickly fatal according to the specimen (Fig. 170). 



The large intestine is the chosen haunt of the bacteria of the 

 alimentary canal; they swarm in the faeces, and by their influence, 

 especially in the caecum of herbivora, but also to some extent in 

 man, even cellulose is broken up, the final products comprising 

 certain fatty acids, such as butyric, acetic and valerianic acids, 

 carbon dioxide and marsh gas. A cellulose-dissolving enzyme of 

 great activity is present in the hepatic secretion of the snail, which 

 rapidly produces sugar from that polysaccharide. De'xtrose is also 

 formed when it is hydrolysed by dilute acid. Apart from the im- 

 portance of solution of the cellulose in facilitating the action of the 

 digestive juices on the starch and other nutrient materials enclosed 

 by it, it can be assumed that some of the intermediate products of 

 its hydrolysis by the bacteria e.g., bodies analogous to the dex- 

 trins which appear in the hydrolysis of starch can be acted on by 

 the ferments of the succus entericus and the pancreatic juice, so 

 as to form dextrose, which on absorption then takes its place in 

 the carbo-hydrate metabolism just as if it had been derived from 



27 



