88 PHYSIOLOGICAL CHEMISTRY 



animal and vegetable, and particularly in the brain and nerve 

 tissues. Little is known of the biological function of these com- 

 pounds, although as our information increases, they appear to 

 be increasingly important. The composition of some of them 

 is known, whereas in the case of others, we have no assurance 

 that the substances reported are single compounds and not mix- 

 tures of closely related compounds. 



The term lipoid has been used to designate these "fat-like" 

 bodies, but the term has been used in so many different con- 

 nections that MacLeaii suggests it be abandoned. He proposes 

 the name "lipins" for the two groups, phosphatids and cere- 

 brosides. These substances, which may be obtained from various 

 tissues by extraction with ether, yield, among other things, 

 fatty acids on hydrolysis. The phosphatids contain both nitro- 

 gen and phosphorus, and on hydrolysis yield phosphoric acid, 

 glycerine, fatty acids, and a nitrogen base such as choline or 

 amino-ethyl alcohol. They are classified on the basis of the rel- 

 ative amounts of nitrogen and phosphorus which they contain. 

 Lecithin, kephalin and sphingomyelin are the best known mem- 

 bers of the group. 



The cerebrosides, obtained chiefly from the brain, contain 

 nitrogen, but no phosphorus. On hydrolysis they yield, among 

 other things the carbohydrate galactose or one of its derivatives. 

 The only two substances definitely known to be individuals in 

 this group, and not mixtures, are phrenosin and kerasin. Little 

 is known of the functions of the cerebrosides, but their presence 

 in the brain is sufficient to make them interesting. 



Lecithin. Lecithin, classed as a phosphatid, is one of the 

 interesting compounds of this type. It is found in all cells, in 

 greatest amount in egg yolk which contains about 10%. 

 Lecithin is soluble in absolute alcohol, and in ether, but may 

 be precipitated from the latter solution by adding acetone. On 

 hydrolysis lecithin yields higher fatty acids, glycerine, phos- 

 phoric acid and an organic base choline. It is believed to have 

 the following formula, where B indicates a fatty acid residue. 



