Outline of a Classification of the Lipoids. 139 



Compound Lipoids. 



Compounds of the fatty acids with alcohols but containing 

 other groups in addition to the alcohol. 



Phospholipoids. — Substituted Fats containing phosphoric acid 

 and nitrogen — lecithin, cephalin, etc. 



Glycolipoids. — Compounds of the Fatty acids with a carbo- 

 hydrate and nitrogen but containing no phosphoric acid. — Cere- 

 bron, etc. 



(Aminolipoids, Sulpholipoids etc. — Various groups which may 

 be added as soon as they are sufficiently well characterized.) 



Derived Lipoids. 



Substances derived from the above groups by splitting, which 

 have the general properties of the lipoids. 

 Fatty Acids of various series. 



Sterols. — Alcohols, mostly large molecular solids, found 

 naturally in combination with the fatty acids and which are 

 soluble in "fat solvents'— Cetyl Alcohol (Ci 6 H 33 OH), Myricyl 

 Alcohol (C 30 H 6 iOH), Cholesterol (C27H43OH), etc. 



Notes on the Classification. 



The group is specifically limited and defined in two ways ; 



1. Only substances are included which are chemically and 

 metabolically related to the fatty acids. 



2. Only naturally occurring substances are included. 



The definitive chemical entity of the group is the fatty acid 

 and it is intended to include only those substances which are 

 closely concerned with the metabolism of the fatty acids. The 

 second limitation — "naturally occurring" is intended to exclude 

 organic compounds which have no relation to the metabolism of 

 the fatty acids but which would otherwise be included owing to 

 composition or physical properties. 



The name "Lipoid" has been chosen for the group because 



1. By derivation it is suitable. 



2. When limited as above it is sufficiently definite for the 

 purpose and yet general enough to include all necessary sub- 

 stances. 



