PHYSIOLOGY OF THE LIVER AND SPLEEN. 811 



way to make new hemoglobin in the hematopoietic organs. Since 

 the hematin constitutes only 4 per cent, of the hemoglobin, it is evi- 

 dent that in the production of the bilirubin a considerable amount 

 of globin must be formed also, but nothing is known of the fate of 

 this portion of the molecule. The general view has been that hem- 

 oglobin liberated from disintegrated red corpuscles serves as the 

 sole source of bile pigment, but some recent observers* have ques- 

 tioned this assumption. They show that the amount of bile pig- 

 ment excreted varies with the nature of the diet, being markedly 

 increased, in dogs, by a carbohydrate diet, and reduced by a pure 

 meat diet. This fact would seem to indicate some relationship 

 between liver activity and pigment formation. The bile pig- 

 ments are carried in the bile to the duodenum and are mixed 

 with the food in its long passage through the intestine. Under 

 normal conditions neither bilirubin nor biliverdin occurs in the 

 feces, but in their place is found a reduction product, urobilin, 

 formed in the large intestine, probably in consequence of the 

 activity of the bacteria. Moreover, it is believed that some of 

 the bile pigment is reabsorbed as it passes along the intestine, is 

 carried to the liver in the portal blood, and is again eliminated. 

 That this action occurs, or may occur, has been made probable 

 by experiments of Wertheimer* on dogs. It happens that sheep's 

 bile contains a pigment (cholohematin) that gives a characteristic 

 spectrum. If some of this pigment is injected into the mesenteric 

 veins of a dog it is eliminated while passing through the liver, and 

 can be recognized unchanged in the bile. The value of this "cir- 

 culation of the bile," so far as the pigments are concerned, is not 

 apparent. 



Bile Acids. — "Bile acids" is the name given to two organic acids, 

 glycocholic and taurocholic, which are always present in bile, and, 

 indeed, form very important constituents of that secretion; they 

 occur in the form of their respective sodium salts. In human bile 

 both acids are usually found, but the proportion of taurocholate 

 is variable, and in some cases it may be absent altogether. 

 Among herbivora the glycocholate predominates, as a rule, although 

 there are some exceptions; among the carnivora, on the other hand, 

 taurocholate occurs usually in greater quantities, and in the dog's 

 bile it is present alone. Glycocholic acid has the formula CagH^jNOg, 

 and taurocholic acid the formula CjeH^sNSO^. Each of them can 

 be obtained in the form of crystals. When boiled with acids or alka- 

 lies these acids take up water and undergo hydrolytic cleavage, the 

 reaction being represented hy the following equations: 



* Whipple and Hooper, "American Journal of Physiology," 40, 349, 1916. 

 t "Archives de physiologic normale at pathologique," 1892, p. 577. 



