XOIOIAL FAT METABOLISM 187 



significance in the consideration of these lipoids as constituents of living 

 matter. 



The chemical formula of a typical lecithin which embodies our 

 knowledge of its composition at the present time is: 



CIT 2 O K! As indicated by the formula 



| the fatty acid groups (ll l and R 2 ) 



are generally different and the com- 



| pounds are optically active. The 



CII E 2 fatty acids are often unsaturated, 



particularly in the lecithins from 

 O the active organs as heart, liver, 



/ etc. 



CH 2 -O-P-OII 



O 



C 2 H 4 ' 



O 



H 



Cepliallns. These differ from lecithins in being difficultly soluble in 

 alcohol and in containing a different basic group, the exact nature of 

 which is unknown, but which is believed to be ammo-ethyl alcohol. They 

 are widely distributed in the body and, according to Thudicum, are the 

 main phospholipoids of the brain. They have recently received a good 

 deal of attention because of their connection with blood coagulation 

 (Howell). MacLean has shown that they are formed rather easily from 

 lecithin and that one of the difficulties in preparing pure lecithin is its 

 tendency to lose its methyl groups and pass over into cephalin. 



Glycolipoids. These substances, characterized by their content of car- 

 bohydrate, are less imderstoocl than the phospholipoids. The only one 

 which has been well studied is the cerebrone of Thierfelder, a constituent 

 of brain tissue. The carbohydrate is galactose, the fatty acid a higher 

 isomer of stearic acid, and there is also a basic substance^ known as 

 sphingosine. 



Derived Lipoids. Fatty Acids. The fatty acids found combined in 

 the fats include practically all the known fatty acids of the various series 

 which contain even numbers of carbon atoms arranged in straight chains. 

 Fatty acids of odd numbers of carbon atoms are so rare that their natural 

 origin is questionable, while branched chains are unknown. A few acids 



