334 ESSENTIALS OF PHYSIOLOGY. 



normally in carnivora, and in man when the diet contains a due 

 proportion of protein, holds calcium phosphate in solution, and in 

 such a case the proportion of calcium excreted by the large intestine 

 is relatively small. When the urine is alkaline, on the other hand, as 

 in herbivora, and in man when the diet is largely vegetable, the amount 

 of calcium excreted by the bowel is greater. Other chemical substances 

 taken as drugs, for example mercury, may also be excreted by the 

 large intestine. 



(3) The only substance absorbed in any quantity in the large bowel 

 is water. The contents of the ascending colon contain no nutritive 

 substances, but their bulk is fairly large owing to the amount of 

 fluid which they contain. During their stay in the large intestine 

 the bulk is greatly reduced, chiefly by the absorption of water. It 

 is said that 400 c.c. are absorbed from the contents of the colon in 

 twenty-four hours. The possibility of the absorption of nutritive 

 substances in the large intestine is of importance, because attempts 

 are frequently made to introduce food-stuffs into the body by means 

 of rectal injections. Experiments prove, however, that nutritive 

 material is not absorbed by the large intestine, with the exception 

 of small amounts of dextrose, which are too minute to be of real 

 practical value. 



(4) The bacteria in the human large intestine act upon cellulose 

 with the production of lower fatty acids, marsh gas (CH 4 ), carbonic 

 acid, and hydrogen. Undigested protein residues also undergo bacterial 

 decomposition with the production of the aromatic bodies, indol (C 8 H 7 N), 

 skatol (methyl-indol), and phenol. It is possible that the fatty acids 

 derived from cellulose may be absorbed, as they are in the herbivora, 

 and there is evidence that absorption of indol, skatol, and phenol takes 

 place, inasmuch as compounds of these substances with sulphuric acid, 

 the ethereal sulphates, are found in the urine. 



THE F-ECES. 



The residues which finally reach the rectum constitute the fseces, 

 and form a solid or semi-solid mass, coloured by the pigment sterco- 

 bilin, which is derived from bilirubin. The composition of the faeces 

 has already been indicated. They contain about 65 per cent, of water, 

 with organic material and inorganic salts. The organic substances 

 are partly nitrogenous, and partly of a fatty nature and soluble in 

 ether. The nitrogenous constituents include cholic acid, dyslysiu, 

 indol and skatol, purin bodies, epithelial cells, and dead bacteria. The 

 lipoids are fatty acids, lecithin, and coprosterin, a body allied to 



