224 PRACTICAL PHYSIOLOGY 



all like solubilities in the various solvents used in extraction. Some 

 are soluble in hot alcohol but not in cold, some in ether and not in 

 acetone, and so on. These varying solubilities are used for the 

 separation and isolation of these lipoid substances. 



Lipoids have been classified in various ways, and probably the 

 simplest is 



,(1) Phosphatides . Substances which contain both phosphorus 

 and nitrogen in varying amount. These substances are sometimes 

 called Phospho-lipins. 



(2) Cerebrosides. Substances which contain nitrogen but no 

 phosphorus. These substances are sometimes called Galactosides 

 or Galacto-lipins*. 



(3) A class of substances which, like neutral fat, contains neither 

 phosphorus nor nitrogen. 



Group I, the phosphatides, has been subdivided by many workers 

 according to the N : P ratio, but recent work would show that at 

 present only three phosphatides can be definitely identified, viz. 

 lecithin, cephalin and sphingomyelin. 



Lecithin is a compound of a base cholin, phosphoric acid, glycerol, 

 and two fatty acids. It is found widely distributed in the various 

 tissues. It is an extremely labile substance. Many functions have 

 been ascribed to it. 



Cephalin contains glycerol, phosphoric acid, two fatty acids and 

 a base aminoethanol. It is mainly found in connection with nerve 

 tissue. Both lecithin and cephalin are extracted by ether, but 

 they are differentiated by the fact that lecithin is soluble in alcohol 

 whereas cephalin is insoluble. 



Sphingomyelin contains no glycerol but phosphoric acid and two 

 fatty acids are present in addition to two bases, cholin and sphin- 

 gosine. It is found in largest amount in brain tissue. 



Group II, the cerebrosides or galactosides, have been much less 

 fully studied than the phosphatides. They are the principal con- 

 stituents of the substance, formerly believed to be an entity, known 

 as protagon, a substance which can be readily extracted from brain 

 tissue with hot alcohol. The best-known member of the group is 

 phrenosin (sometimes called cerebron). The cerebrosides yield on 

 decomposition one fatty acid, the base sphingosine and a reducing 

 sugar, the monosaccharide galactose. 



Cholesterol. Cholesterol is the substance selected to exemplify 

 Group III. Although soluble in the same solvents as fats and the 

 lecithins, cholesterol is not a fat, but an unsaturated secondary 

 alcohol belonging to the terpene series. The terpenes are common 

 in plants, examples of them being camphor and turpentine. 



Like the lecithins it is very widely distributed in the animal 

 body. In the free state, it is present in the envelope and stroma 

 of the red blood corpuscles ; both as an ester, and in the free 

 state it is present in the blood. It is also present in bile, and it 

 may separate out from this to form calculi . A variety of cholesterol, 



