562 METABOLISM, NUTRITION AND DIETETICS 



is probably split, with liberation of the fatty acid, in the course of 

 digestion. But if this be so, cholesterin esters are again formed 

 in the tissues, for the cells and the blood contain both cholesterin 

 esters and free cholesterin. While some cholesterin is excreted in 

 the faeces (p. 419), there is evidence that a portion of the cholesterin 

 of the bile may be reabsorbed, a ' circulation ' of cholesterin taking 

 place analogous to the circulation of bile-salts. The appearance of 

 cholesterin in the bile has been connected by some writers with the 

 destruction of erythrocytes in the liver, or the conveyance of the 

 products of their decomposition to that organ (p. 21), but there 

 are no means of distinguishing between the cholesterin set free from 

 blood-corpuscles and that liberated from other cells. Since it is 

 contained in all cells, every cell may be supposed to contribute 

 something from time to time to the cholesterin excretion. 



As to the office of the tissue-cholesterin, it can only be suggested 

 that a substance so ubiquitous must be important. There is some 

 evidence that cholesterin, free or combined, plays a part in con- 

 ferring on the cells those peculiarities in their permeability upon 

 which their functions, and indeed their integrity, depend. Free 

 cholesterin, for instance, hinders the haemolytic action of the 

 saponins (p. 28), apparently by forming compounds with them. 

 Whether it or its esters are actually concentrated at the surface of 

 the cell, and contribute to the formation there of the so-called 

 ' lipoid ' envelope, is not definitely known, although there are facts 

 in favour of this idea. 



Metabolism of Phosphatides. The lecithins, which are the best- 

 known members of this class of compounds, have been already 

 described (p. 360). They are built up of glycerin, fatty acids, 

 phosphoric acid in the form of glyceryl-phosphoric acid, and a 

 nitrogenous base cholin. There is some reason to think that the 

 lecithins of the tissues are, in part at least, not free, but combined 

 with proteins or with carbo-hydrates. Other bodies belonging to 

 the phosphatide group are kephalin, a constituent of nervous tissue 

 and of yolk of egg, cuorin found in heart muscle, etc. 



It is probable, as stated in the chapter on Digestion, that the 

 phosphatides of the food are hydrolysed in the alimentary canal 

 with liberation of the glycerin, fatty acids, and the other com- 

 ponents. It is not known whether they are resynthesized in the 

 intestinal wall, but it is more probable that they pass directly to 

 the tissues, where they can be utilized for building up the phos- 

 phatides of the cells. Cholin is found in small quantities free in the 

 tissues, and also, it is said, in the blood-plasma. Glyceryl-phos- 

 phoric acid has also been obtained in small amount from various 

 tissues. The other components of lecithin are, of course, never 

 wanting, and there can be no doubt that the cells possess the power 

 of reconstructing phosphatides from'such materials. They can do 



