PHYSIOLOGY OF THE LIVER AND SPLEEN, 813 



CH2SHCHNH2COOH + 30 = CH2SO2OHCHNH.2COOH; 



Cystein. Cysteinic acid. 



and by loss of CO2 this is converted to taurin : 



CH2SO2OHCHNH2COOH — CO2 = CH2SO2OHCH2NH2. 



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 Idv tlie 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, *^hey serve as a menstruum 

 for dissolving the cholesterin which is constantly present in the bile; 

 secondly, they facilitate greatly 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 chiefiy to the presence of the bile-acids, and 

 in the same way the known activating infiuence of bile upon the 

 activity of pancreatic lipase has been traced to the bile-acids. The 

 bile-acids, the taurocholate, at least, possess the property of pre- 

 cipitating proteins in acid solutions. This property probably ex- 

 plains the fact that the acid chyme as it passes into the duodenum 

 is precipitated by coming into contact with the bile, a fact which 

 has long been known, although its physiological significance is not 

 clear. 



Cholesterin or Cholesterol. — Cholesterin is a non-nitrogenous 

 substance of the formula C27H4eO. (See p. 80.) It is a constant 

 constituent of the bile, although it occurs in variable quantities. 

 Cholesterin is very widely distributed in the body, being found 

 especially in the white matter (medullary substance) of nerve- 

 fibers. It seems, moreover, to be a constant constituent of all 

 animal and plant cells. It is assumed that cholesterin 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. This is at least a possible explanation of its occur- 

 rence in the bile, for it seems certain that the cholesterin is a 

 constant constituent of the blood, either as such or in the form 

 of an ester, that is to say, in combination with a fatty acid, such as 

 stearic acid. Some authors suggest, however, that in the disso- 



