PHYSIOLOGICAL CHEMISTRY 317 



phosphoric acid, which, on the other hand, has attached to it an 

 ammonium base, cholin. The two molecules of fatty acid are usually 

 of the same kind, but they may be different. 



The structural formula for a typical lecithin is therefore : 

 fCH 2 -OOC(CH 2 ) 16 CH 3 ^ 



V Stearic acid. 



H- 



CH-OOC(CH 2 ) 16 CH 3 



Glycerine. 



Phosphoric ^CH 2 CH 2 OH. 



acid - Cholin. 



Cholin is closely related chemically to certain basic bodies occurring 

 in plants, one of which is muscarin, an alkaloid with a strong pharma- 

 cological action on the heart and glands. Cholin itself has a pro- 

 nounced pharmacological action, thus, it produces a marked fall in 

 blood pressure. In the free state cholin is not present in the blood, 

 for it appears that the cholin liberated in the intestine by the break- 

 down of lecithin is destroyed before absorption. During degeneration 

 of nervous tissue, of which lecithins are important constituents, cholin 

 may appear in a free state in the blood; in such cases it can be 

 recognised by adding platinic chloride to an alcoholic extract of blood, 

 or cerebro-spinal fluid, when crystals of choline platinochloride separate 

 out. These are yellow in colour and octahedral or prismatic in shape. 

 Similar crystals are produced by adding platinic chloride to ammonium 

 or potassium chloride, but the cholin crystals can be distinguished from 

 those by adding a strong solution of iodine in potassium iodide, when 

 the choline crystals become changed into dark brown plates, which 

 afterwards change into oily droplets. 



From their chemical structure, we see that the lecithins, besides 

 being closely related to fats, bear some relationship to the nucleins; 

 both contain phosphorus, and it has been suggested that the phosphorus 

 of nuclein is derived from the phosphorus of lecithin. The lecithins 

 can also form various combinations with proteins. These are some- 

 times called lecithides. The lecithins are split up by lipase and 

 possibly reconstructed in the various tissues in which they are found 

 present. Further indication of their importance in the animal economy 

 is found in the fact that they can act on the so-called complement in 

 the laking of red blood corpuscles by such substances as snake venom. 

 They are important constituents of the cell wall, and have therefore to 

 do with the process of absorption into the cell. 



The lecithins can be recognised chemically by their decomposition 



