FATS AND OILS, WAXES, AND LIPOIDS 141 



proportions in nerve and brain substance, in egg yolk, etc., and in 

 the seeds of plants. When hydrolyzed, they yield fatty acids or 

 derivatives of fatty acids and some other group containing either 

 nitrogen only or both nitrogen and phosphorus. The facts that 

 they are extracted from tissues by the same solvents which extract 

 fats and that they yield fatty acids when hydrolyzed account for 

 the name " lipoid," which comes from the Greek word meaning 

 fat. Some writers, who object to the word " lipoid " as a group 

 name, prefer to call these substances the " fat-like bodies." 



The first group of lipoids to be studied were those which occur 

 in the brain; and the name cerebroside was given to those lipoids 

 which, when hydrolyzed, yield fatty acids, a carbohydrate and a 

 nitrogen-containing compound but no phosphoric acid; while 

 those lipoids which contain both nitrogen and phosphorus were 

 called phosphatides. Substances which correspond in composition 

 to both these types are found in plant tissues and the same class 

 names are applied in a general way to lipoids of either plant or 

 animal origin. 



Plant lipoids have not been studied to nearly the same extent 

 as have those which occur in the animal body; and certain observ- 

 ers believe that there are significant differences between the lipoids 

 of plants and those of animal origin. However, most investigators 

 use the same methods of study and the same systems of nomen- 

 clature for these fat-like substances, regardless of their origin. 



LECITHIN 



This phosphatide is by far the best-known lipoid. It occurs in 

 the brain, the heart, the liver, and in the yolk of the eggs of many 

 animals; and either lecithin or a substance so nearly like it in 

 character as to be regarded by most investigators as identical with 

 it, is present in small, but constant, quantities in nearly all seeds, 

 especially those of leguminous plants. In many legume seeds, it 

 constitutes from 50 to 60 per cent of the " ether extract," or 

 " crude fat," which can be extracted from the crushed seeds, using 

 ether as the solvent. 



Lecithin is a glyceride. Only two of the (OH) groups of the 

 glycerol are replaced .by fatty acids, however; the third being 

 replaced by phosphoric acid, HsPO^ or PO(OH)s, which, in turn, 

 has one of its hydrogen atoms replaced by the base choline. Cho- 

 line is a nitrogenous base, or amine, which may be regarded as 



