THE CHOLESTERINS. 67 



the yolk of eggs it occurs in combination with vitellin, but is here 

 apparently not closely bound. A certain similarity thus exists 

 between the lecithins and the nucleins ; both contain phosphorus 

 in their molecules, and both combine with albumins to form more 

 complex substances. The lecithins occur widely distributed in both 

 the animal and vegetable world. According to Hoppe-Seyler, they 

 are found in all cells and bodily fluids. They are especially abun- 

 dant in nerve-tissue and also in the eggs and semen of most animals. 

 Their isolation and special tests will be considered in a subsequent 

 chapter. 



THE CHOLESTERINS. 



The cholesterins are monatomic alcohols of the formula C^H^. 

 OH -f- H 2 O, and occur widely distributed both in the vegetable and 

 the animal world. They are especially abundant in nerve-tissue 

 and in the bile. In the gall-bladder they are frequently found in 

 the form of so-called gall-stones, and not uncommonly constitute 

 the greater portion of their solids. Different varieties apparently 

 exist, such as the common cholesterin of the concretions just men- 

 tioned, isochoiesterin (which has been obtained from lanolin), phy- 

 tosterin, paracholesterin, and kaulosterin, which are found in 

 plants. Their structural composition is unknown. Like the fats, 

 they are insoluble in water, but soluble in ether, alcohol, and chloro- 

 form, from which solutions they may be obtained either in the form 

 of very characteristic platelets or as needle-like crystals. In solu- 

 tions of the alkalies, in the absence of alcohol, they are entirely 

 insoluble, even on boiling, in which respect they differ from the fats. 

 Like glycerin, cholesterin combines with fatty acids to form com- 

 pound ethers, and in this form it is frequently found in nature. In 

 wool-fat, for example, it is thus present in large amounts, and from 

 it such ethers can be readily obtained. In pure form they constitute 

 the lanolin of the shops. These ethers show a remarkable difference, 

 as compared with the fats, in their behavior toward water. Of this 

 they apparently take up one-quarter of their own weight, and on 

 stirring give rise to a pasty, frothy mass. 



From their ethereal compounds cholesterin can readily be sepa- 

 rated by treating with diacetic- ethyl ether, which dissolves the 

 cholesterin and leaves the ethers behind. Their special tests, and 

 also their mode of isolation, will be taken up in a subsequent 

 chapter. 



After having thus studied the three great classes of food-stuffs 

 which plants are capable of elaborating from water, carbon dioxide, 

 and certain mineral salts, and which are also represented in the 

 animal body, we shall now proceed to a similar survey of the natural 

 decomposition-products of these substances which are formed during 

 their passage through the animal body, and which are of more or less 

 interest in indicating the manner in which these decompositions 



