58 PHYSIOLOGY 



form, insoluble in absolute alcohol. The universal distribution of lecithin 

 seems to indicate that it plays an important part in the metabolic processes 

 of the cell. There is no doubt that it may serve, inter alia, as a source of 

 the phosphorus required for building up the complex nucleo-proteins of cell 

 nuclei. It seems to represent an intermediate stage in the utilisation of neu- 

 tral fats by protoplasm, and its occurrence in the brain as a constituent of 

 more complex molecules, which contain also a carbohydrate nucleus (galacto- 

 sides, such as cerebrin), might be interpreted as indicating some share also in 

 the metabolism of carbohydrates. 



Lecithin may be extracted from tissues by boiling with absolute alcohol. 

 On cooling the alcoholic extract in a freezing mixture, the lecithin separates 

 out as granules or semi- crystalline masses. When dried in vacua, it forms 

 a waxy mass, which melts at 40 to 50 C. In water it swells up to form a. 

 paste, which, under the microscope, is seen to consist of oily drops or threads, 

 the so-called myelin droplets. In a large excess of water it forms an emulsion 

 or a colloidal solution. Its power of taking up water on the one hand, and 

 its solubility in alcohol and similar media on the other, give it an intermediate 

 position between the water-soluble crystalloids and the insoluble fats, and 

 enable it to play an important part both as a vehicle of nutritive substances 

 and as a constituent of the lipoid membrane, which bounds and determines 

 the osmotic relationships of all living cells. 



The phospholipines are provisionally classified according to the proportions of 

 N and P in their molecule, as follows : 



(a) Mono-amino-monophosphatides, N : P = 1 : 1 (including lecithin and cephalin). 

 (6) Diamino-mono-phosphatides, N : P = 2 : 1 (e.g. sphingomyelin). 



(c) Mono-amino diphosphatides, N : P = 1 : 2 (e.g. cuorin, a lipine extracted from 



heart muscle by Erlandsen). 



(d ) Diamino-diphosphatides, N : P = 2 : 2. 



(e) Triamino-monophosphatide, N : P = 3 : 1 (an example has been reported as 



occurring in egg yolk). 



All these bodies (except cuorin) are obtained by the extraction of the brain or of nerve 

 fibres. Many also occur in egg yolk. The galacto-lipines include two substances 

 extracted from the brain, viz. phrenosin and kerasin. Both these on decomposition 

 yield galactosc, a nitrogenous base called sphingosine and a fatty acid. We know 

 little or nothing of their significance. 



