SURVEY OF DIGESTION AS A WHOLE 423 



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. Dextrose 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 

 starch. In the herbivora the contribution thus made to the nutri- 

 tion of the animal may be of considerable importance; in omnivora 

 it is not negligible. In man as much as 40 per cent, of the cellulose 

 of young vegetables is said to be capable of assimilation. In car- 

 nivorous animals, however, it appears that cellulose when taken in 

 the food is quantitatively excreted in the faeces. In addition to 

 the action of the intestinal flora on cellulose, certain of the bacteria 

 of the alimentary canal affect some of the other carbo-hydrates in a 

 not unimportant way. Dextrose, for instance, can be decomposed 

 into two molecules of lactic acid, according to the equation 



C 6 H 12 O 6 = 2C 3 H 6 O 3 . 



This is called the lactic acid fermentation, and is due to a special 

 bacillus. 



Another micro-organism splits up dextrose into butyric acid, 

 carbon dioxide and hydrogen, the so-called butyric acid fermenta- 

 tion, according to the equation 



C 6 H 12 O 6 - C 4 H 8 O 2 + 2CO 2 + 2H 2 . 



The contents of the large bowel are generally acid from the 

 products of bacterial action, although the wall itself is alkaline. 

 Faeces. In. addition to mucin, secreted mainly by the large 



