78 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



is composed largely of bodies to which the general name of " li- 

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

 those constituents of living cells which can be extracted by ether 

 or similar solvents. It is a biological rather than a chemical 

 term. By extraction of myelin with hot alcohol a complex phos- 

 phorus-containing substance known as protagon may be obtained 

 in crystalline form. This substance is, however, believed now to 

 be a mixture rather than a definite chemical individual. The 

 most important substances isolated from the myelin are lecithin 

 (phosphatids) , cholesterin, and the cerebrosides. 



Lecithin (C 4 4H9oNPO 9 ) is a waxy hygroscopic yellowish sub- 

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

 composed by the action of alkalies it yields as split products 

 glycerophosphoric acid, a nitrogenous base, cholin (C 5 H 15 NO 2 ), 

 and some of the higher fatty acids, such as oleic, palmitic, 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. 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 shown 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 (C^H^O) 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 

 to the group of "terpenes" heretofore supposed to be confined 

 to the plant kingdom. It is given the formula 

 CH-CH 2 -CH 2 -C 17 H 2fl -CH 



CHOH 



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

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

 Nerve/' Philadelphia, 1904. 



