CLASSIFICATION 3 



form, hot alcohol, benzene, carbon tetrachloride, acetone, etc.) which are 

 iKelated .either -act^alty or potentially-to fatty -acid- .esters, and which at the 

 .same time areutilizable by the animal organism.- The latter qualification 

 as essential to exclude minel'al oil derivativ-es. Thi« definition should not be 

 interpreted too rigorously, or it would exclude certain compounds long x;ion- 

 sider^d. to'be members- of this group. Thus, lecithin is to some extent 

 water-soluble; lecithin is insoluble in acetone, cephalin in alcohol, while the 

 sphingomyelins and the cerebrosides are insoluble in such a widely accepted 

 fat solvent as diethyl ether. 



The terms fats and oils are generally considered to refer to substances 

 which have a similar chemical structure, and which have the same metabo- 

 lism in the animal body. Fats are those substances which are solids at 

 ordinary temperatures, while oils are liquids under similar circumstances. 

 Thus, what may be classed as a fat in one locality may be considered as an 

 oil in a warmer climate. 



3. Classification 



The most convenient classification and the one which has been most 

 widely employed in this country is the one originally suggested by Bloor.'^ 

 This is as follows : 



(1) Simple Lipids 



a. Neutral Fats. These are glycerol esters (glycerides) of the fatty 

 acids (chiefly palmitic, stearic, oleic, and linoleic). 



b. Waxes. These are esters of monatomic alcohols higher than glyc- 

 erol. Most of these are substances with high melting points which are 

 hydrolyzable with difficulty. These may be divided into several sub- 

 classes as follows: 



(a) True Waxes. These consist of products of both animal and plant 

 origin in which the esters are composed of palmitate, stearate, oleate, or 

 other higher fatty acid esters of cetyl (CH3(CH2)i4CH20H), octadecyl or 

 steryl (CH3(CH2)i6CH20H), or other higher straight-chain alcohols. 



(6) Cholesterol Esters. These are fatty acid esters of the sterol, choles- 

 terol. Lanolin, which contains a mixture of cholesterol palmitate, stea- 

 rate, and oleate, is a member of this group. 



(c) Vitamin A Esters. Vitamin A occurs naturally as the palmitic or 

 stearic acid ester. In contradistinction to the waxes, these compounds are 

 readily hydrolyzable. The colored fats which are mono- or di-esters of the 

 carotenols, also, should be included in this category. 

 ^ (d) Vitamin D Esters. Since vitamin D occurs naturally as an ester, it 

 also should be included as a part of the group of waxes. 



' W. R. Bloor, Chem. Revs., 2, 243-300 (1925-1926). 



