CHEMISTRY OF DIGESTION AND NUTRITION. 323 



from haemoglobin. This origin was first indicated by the fact that in old 

 blood-clots or in extravasations there was found a crystalline product, the 

 so-called " haematoidin," which was undoubtedly derived from haemoglobin, 

 and which upon more careful examination was proved to be identical with 

 bilirubin. This origin, which has since been made probable by other reac- 

 tions, is now universally accepted. It is supposed that when the blood- 

 corpuscles go to pieces in the circulation (p. 45) the haemoglobin is brought to 

 the liver, and then, under the influence of the liver-cells, is converted t<> an 

 iron-free compound, bilirubin or biliverdin. It is very significant to find that 

 the iron separated by this means from the haemoglobin is for the most part 

 retained in the liver, a small portion only being secreted in the bile. It seems 

 probable that the iron held back in the liver is again used in some way to 

 make new haemoglobin in the haematopoietic organs. The bile-pigments 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 is found in the feces, but in their place is found a reduction pro- 

 duct, hydrobilirubin, formed in the large intestine. 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 "I 

 Wertheimer l on dogs. It happens that sheep's bile contains a pigment 

 (cholohsematin) that gives a characteristic spectrum. II' some of this pig- 

 ment 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 "circulation of the bile," so far as the pigments are con- 

 cerned, is not apparent. 



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

 eholic 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 this latter acid 

 may be absent altogether. Among herbivora the glyeocholate predominates 

 as a ride, although there are some exceptions ; among the carnivora, on the 

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

 bile it is present alone. Glycoeholic acid has the formula ( ' J . |;! N( >,.,, and 

 taurocholic acid has the formula CoJI^NSOj. Each of them can be obtained 

 in the form of crystals.' When boiled with acids or alkalies these acids take 

 up water and undergo hydrolytic cleavage, the reaction being represented by 

 the following equations: 



c 2S h 43 no 6 + h 2 o = C 24 H 40 O, + ch 2 (NH 2 )COOH. 



Glycoeholic acid. Cfaolic acid. Glycocoll (amido-acetic add). 



C 26 H 45 NS0 7 + H 2 34 H 4() O, + fVH.XIl ,S( )...( )U 



feurocholic acid. Cholicacid. Taurin (amido-ethyl 



Bulphonic acid 



1 Archives de Physiologie normale ei paihologique, 1892, p. 577. 



