726 PHYSIOLOGY OF DIGESTION AND SECRETION. 



than in the liver. It is more difficult to ascertain from what sub- 

 stances they are formed. The fact that glycocoll and taurin contain 

 nitrogen, and that the latter contains sulphur, indicates that some 

 proteid or albuminoid constituent is broken down during their pro- 

 duction. 



From the standpoint of nutrition the taurocholate is interesting as giving 

 one of the forms in which the sulphur of proteid material is eliminated. Some 

 light has been thrown upon the origin of taurin by the discovery (Friedmann*) 

 that it may be formed from cystin. This latter body, C 6 H 12 N 2 S 2 4 , is known 

 to occur as one of the end-products in the acid hydrolysis of proteids, and it 

 is possible that it occurs also in the tryptic-erepsin hydrolysis in the small 

 intestine, representing the end-product in which the sulphur of the proteid 

 molecule is found. Cystin may be oxidized to cysteinic acid (COOHC 2 H 3 NH 2 - 

 S0 2 OH) and from this taurin (C 2 H 4 NH 2 S0 2 OH) may be obtained. It is prob- 

 able, therefore, that the taurin is formed normally from cystin in the body 

 and that the latter represents one of the split products of proteid. f Some 

 of the sulphur of the cystin appears also in the urine in oxidized form as sul- 

 phate. Under certain pathological conditions the cystin itself appears in the 

 urine, giving the phenomenon of cystinuria. 



A circumstance of considerable physiological significance is that 

 these acids or their decomposition products are absorbed in part from 

 the intestine and are again secreted by the liver; as in the case of the 

 pigments, there is an intestinal-hepatic circulation. The value of this 

 reabsorption may lie in the fact that the bile acids constitute a very 

 efficient stimulus to the bile-secreting activity of the cells, being one 

 of the best of cholagogues, or it may be that it economizes material. 

 From what we know of the history of the bile acids it is evident that 

 they are not to be considered solely as excreta: they have some 

 important function to fulfill. The following suggestions as to their 

 value have been made : In the first place, they serve as a menstruum 

 for dissolving the cholesterin which is constantly present in the bile 

 and which is an excretion to be removed; secondly, they facilitate 

 the splitting and the absorption of fats in the intestine. It is an 

 undoubted fact that when bile is shut off from the intestine the 

 absorption of fats is very much diminished, and it has been shown 

 that this action of the bile in fat absorption is due to the presence 

 of the bile acids. 



Cholesterin. Cholesterin is a non-nitrogenous substance of the 

 formula C 27 H 46 0. It is a constant constituent of the bile, although 

 it occurs in variable quantities. Cholesterin is very widely distrib- 

 uted in the body, being found especially in the white matter (medul- 

 lary substance) of nerve fibers. It seems, moreover, to be a constant 

 constituent of all animal and plant cells. It is assumed that choles- 

 terin is not formed in the liver, but that it is eliminated by the liver 

 cells from the blood, which collects it from the various tissues of the 

 body. According to Naunyn, however, the cholesterin is not secreted 



* Friedmann, " Hofmeister's Beitrage," 3, 1, 1902. 



t See Simon, " Johns Hopkins Hospital Bulletin, " 15, 365, 1904. 



