320 AN AMERICAN TEXT- HOOK OE PHYSIOLOGY. 



increased by an increase of the fats in the fond. (3) Products of the intes- 

 tinal secretions. Evidence lias accumulated in recent years 1 to show that 

 the l'rcv> in man on an average diet arc composed mainly of the material of 

 the intestinal secretions. The nitrogen of the feces, formerly supposed to 

 represent undigested food, seems rather to have its origin in these secretions, 

 and, therefore, like tin- nitrogen of the urine represents so much metabolism 

 in the body. (4) Products of bacterial decomposition. The most character- 

 istic of these products arc 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 CJLX, 

 and skatol, which is a methyl indol. the formula Cdf.X. They are crystal- 

 line bodies possessing a disagreeable fecal odor; this is epecially true of 

 skatol, to which the odor of the i'vfc< is mainly due. Indol and skatol 

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

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

 through the urine — indol as indican (indoxyl-sulphuricacid), 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). (5) Cholesteriu, 

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

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

 C 78 H 156 S0 2 has been assigned, is found in minute quantities. (7) Mucus and 

 epithelial cells thrown off from the intestinal wall. (8) 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 bilirubin by reduction in the large intestine. (9) Inorganic salts — salts 

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

 the calcium anil iron salts will be referred to in a subsequent chapter, 

 when speaking of their nutritive importance. (10) Micro-organisms. Great 

 quantities of bacteria of different kinds are found in the W'cv^. 



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

 quantity of gas that 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,, H, X, 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. 



F. Physiology of 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 arc seemingly uniform in structure throughout the whole 

 Bubstance 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 



1 See Prausnitz: Zeit&chrift fur Biologie, 1897, Bd. 35, S. 335; and Tsuboi : Ibid., S. 08. 



