80 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



poids" has been applied. This term is used as a generic name tor 

 those constituents of living cells which are fat-like in the sense that 

 they possess solubihties similar to those of the fats. They are ex- 

 tracted by ether or similar solvents. The term applies, therefore, 

 to a somewhat heterogeneous group of substances which have no 

 close chemical relationship to one another. The most important 

 substances isolated from the myehn are lecithin (phosphatids), 

 cholesterin, and the cerebrosides. 



Lecithin (C44H90NPO9) is a waxy hygroscopic yellowish sub- 

 stance containing about 4 per cent, of phosphorus. When de- 

 composed by the action of alkahes it yields as split products 

 glycerophosphoric acid, a nitrogenous base, chohn (C5H15NO2), 

 and some of the higher fatty acids, such as oleic, pahnitic, or 

 stearic. The lecithins constitute one member of a larger group 

 known as phosphatids, which are characterized by the presence of 

 both phosphorus and nitrogen. Other members of the group are 

 cephali)/,, found in the brain, but quite widely distributed in other 

 tissues; cuorin, obtained from heart-muscle, etc. They differ 

 from one another in the character of the nitrogenous base, in the 

 nature of the constituent fatty acids, and in the ratio of the nitrogen 

 to the phosphorus in the molecule. A provisional classification is 

 based on this latter relationship. Those phosphatids in which there 

 is one amino group to each atom of phosphorus are designated as 

 monamino monophosphatids. Lecithin and cephalin, the two 

 best known phosphatids, belong to this group. They are widely 

 distributed in the tissues and liquids of the body, but are especially 

 characteristic of the white matter of the nervous system. They 

 combine easily with other substances, such as proteins, glucosides, 

 etc., and it is probable that lecithin exists in some such combination 

 in the myelin. The decomposition of the lecithin referred to above 

 occurs in the body when nerves undergo degeneration. The 

 presence of the fatty acid liberated under such circumstances is 

 demonstrated by the well-known reaction with osmic acid used to 

 detect degenerated nerve fibers, while the existence of cholin has 

 been sho^vn by Halliburton* in the liquids of the body, not only 

 after nerve-degeneration produced by experimental lesions, but 

 in the case of degenerative diseases of the nervous system. 



Cholesterin or cholesterol (C27H46O) is a white crystalline sub- 

 stance containing, as its formula shows, neither nitrogen nor phos- 

 phorus. It is widely distributed among the tissues of the body, 

 and in an isomeric form, phytocholesterin, occurs also in plants. 

 In the animal body it is especially abundant in the white matter 

 of the nerves. The chemical nature of cholesterin has long been a 

 matter of uncertainty, but recent work indicates that it belongs 

 * Halliburton, "British Medical Journal," 1907, May 4 and 11. Also 

 "Folia Neuro-Biologica," 1907, i., 38, and "Biochemistry of Muscle and 

 Nerve," Philadelphia, 1904. 



