260 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



increased by an increase of the fats in the food. (3) Products of bacterial 

 decomposition. The most characteristic of these products are indol and 

 skatol. These two substances are formed normally in the large intestine from 

 the putrefaction of proteid material. They occur always together. Indol has 

 the formula C 8 H 7 N, and skatol, which is a methyl indol, the formula C 9 H 9 N. 

 They are crystalline bodies possessing a disagreeble fecal odor ; this is espe- 

 ially true of skatol, to which the odor of the feces is mainly due. Indol and 

 skatol are eliminated from the body only in part in the feces ; a certain propor- 

 portion of each is absorbed into the blood and is eliminated in a modified form 

 through the urine indol as indican (indoxyl-sulphuric acid), from which indigo 

 was formerly made, and skatol as skatoxyl-sulphuric acid (see Chemical section 

 for further information as to the chemistry of these bodies). (4) Cholesteriu, 

 which is found always in small amounts and is probably derived from the bile. 

 (5) Excretin, a crystallizable, non-nitrogenous substance to which the formula 

 C r8 H 156 SO 2 has been assigned, is found in minute quantities. (6) Mucus and 

 epithelial cells thrown off from the intestinal wall. (7) Pigment. In addition 

 to the color due to the undigested food or to the metallic compounds contained 

 in it, there is normally present in the feces a pigment, hydrobilirubin, derived 

 from the pigments (bilirubin) of the bile. Hydrobilirubin is formed from 

 the bilirubiu by reduction in the intestine. (8) Inorganic salts salts of 

 sodium, potassium, calcium, magnesium, and iron. The importance of the calcium 

 and iron salts will be referred to again in a subsequent chapter, when speaking 

 of their nutritive importance. (9) Micro-organisms. Great quantities of bac- 

 teria of different kinds are found in the feces. 



In addition to the feces, there is found often in the large intestine a 

 quantity of gas which may also be eliminated through the rectum. This gas 

 varies in composition. The following constituents have been determined to 

 occur at one time or another: CH 4 , CO 2 , H, N, H 2 S. They arise mainly 

 from the bacterial fermentation of the proteids, although some of the N may 

 be derived from air swallowed with the food. 



P. PHYSIOLOGY OP THE LIVER AND THE SPLEEN. 



The liver plays an important part in the general nutrition of the body ; its 

 functions are manifold, but in the long run they depend upon the properties 

 of the liver-cell, which constitutes the anatomical and physiological unit of the 

 organ. These cells are seemingly uniform in structure throughout the whole 

 substance of the liver, but to understand clearly the different functions they 

 fulfil one must have a clear idea of their anatomical relations to one another 

 and to the blood-vessels, the lymphatics, and the bile-ducts. The histology of 

 the liver lobule, and the relationship of the portal vein, the hepatic artery, and 

 the bile-duct to the lobule, must be obtained from the text-books upon histol- 

 ogy and anatomy. It is sufficient here to recall the fact that each lobule is 

 supplied with blood coming in part from the portal vein and in part from the 

 hepatic artery. The blood from the former source contains the soluble prod- 

 ucts absorbed from the alimentary canal, such as sugar and proteid, and these 



