$6 A TEXTBOOK OF PHYSIOLOGY 



SECTION II. 



Lipoids. Under this name are grouped a number of fat-like (lipoid) 

 bodies, resembling fat mainly in their common solubility in certain 

 solvents. They occur in the protoplasm of all cells, and are probably 

 bodies of wide biological significance. They are particularly abundant 

 in nervous tissue. They may be classified as follows : 



I. The Phosphatides bodies containing carbon, hydrogen, oxygen, 

 nitrogen, and phosphorus. 



II. The Galactosides bodies containing carbon, hydrogen, oxygen 

 and nitrogen. 



III. The Cholesterols bodies containing carbon, hydrogen, and 

 oxygen. 



I. THE POHSPHATIDES are all compounds of the triatomic alcohol 

 glycerine, to which a fatty acid, phosphoric acid, and a nitrogenous 

 base are attached. The chief members of this group are the lecithins. 

 The members may be classified according to the proportion of nitrogen 

 to phosphorus in their molecule: 



Monamino-monophosphatides: lecithins, cephalin. 



IN IP 



Monamino-diphosphatides : cuorin. 



IN 2P 



Diamino-monophosphatides : sphingomyelin, jecorin. 



2N IP 



Diamino-diphosphatides, etc. 



2N 2P, etc. 



. Triamino-monophosphatides : carnaubin, obtained from kidney 

 fat, belongs to this group, but yields no phosphoric acid. 



The acticm of narcotics and of chloroform and ether has teen 

 attributed to the solubility of these substances in the phosphatides of 

 the nerve cells. In the reactions of specific cell poisons, toxins such 

 as snake poison, and haemolysins, lecithin may take an active part. 

 So, too, in the action of ferments. Cholesterol, on the other hand, 

 may act as an antibody, precipitating ferments, and neutralizing the 

 action of snake venom. 



The Lecithins. The lecithins are the most important members 

 of. this group of phosphatides, and also, perhaps, the most important 

 of lipoids. T,hey are present in all the cells of the body, and are 

 particularly plentiful in the envelope of red blood-corpuscles, in 

 nervous tissue, bone marrow, liver, and bile. 



The lecithins probably play an important part in the metabolism 

 of the cells, forming easily dissociable compounds with sugars, pro- 

 teins, etc., and helping in the transport of these substances in the body 

 fluids. The fact that each tissue has its specific lipoid helps to endow 

 that tissue with its specific function. In the nervous tissues these 

 substances are of paramount importance, and on extensive degenera- 

 t ion of nervous tissue the body becomes poorer in lecithins. 



