396 A MANUAL OF PHYSIOLOGY 



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

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

 ence, especially in the caecum of herbivora, but also to a small 

 extent in man, even cellulose is broken up, the final products 

 being 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 cellulose. But 

 in herbivorous mammals no such ferment has been found ; and 

 although cellulose can be split up by bacteria in their intestines, 

 sugar is not among the products. In this case the cellulose 

 makes only an insignificant contribution to the metabolism of 

 the animal. 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-pig- 

 ments. Stercobilin is identical with urobilin, which forms a 

 common, though not an invariable, constituent of bile itself. A 

 portion of it is absorbed by the intestine and then excreted in 

 the urine, the urobilin in which is often much increased in fever 

 (' febrile ' urobilin). No bilirubin or biliverdin occurs in normal 

 faeces, although pathologically they may be present. The dark 

 colour of the faeces with a meat diet is due to haematin and sulphide 

 of iron, the latter being formed by the action of the sulphuretted 

 hydrogen which is constantly present in the large intestine on the 

 organic compounds of iron contained in the food or in the secre- 

 tions of the alimentary canal. A small amount of altered bile- 

 acids and their products is also found ; and in respect to these, 

 and to the altered pigments, bile is an excretion. And although 

 its entrance into the upper instead of the lower end of the in- 

 testine, the ascertained importance of its function in digestion, 

 and the fact that the greater part of the bile-salts is reabsorbed, 

 show that in the adult it is very far from being solely a waste 

 product, the equally cogent fact, that the intestine of the new- 

 born child is filled with what is practically concentrated bile 

 (meconium), proves that it is just as far from being purely a 

 digestive juice. Skatol and other bodies, formed by putrefactive 

 changes in the proteins of the food, are also present in the taeces, 

 and are responsible for the faecal odour. Masses of bacteria are 



