4 i8 DIGESTION 



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 fceces. 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 



Fig. 170. Effect of Extract of Faeces on Blood-Pressure. The extract was injected 

 at 2. Time -trace, seconds. 



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

 into two molecules of lactic acid, according to the equation 



C 6 H 12 6 =2C 3 H 6 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 6 = C 4 H 8 2 + 2 C0 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 

 intestine, the faeces consist of indigestible remnants of the food, such 

 as elastic fibres, spiral vessels of plants, and in general all vegetable 

 structures chiefly composed of cellulose. They are coloured with a 

 pigment, stercobilin, derived from the bile-pigments. Stercobilin 

 is identical with urobilin, which forms a common, though not an 



