166 PHYSIOLOGY OF NERVE [CH. XIV. 



they will be more fully studied in Chapter XXVIII. ; we will therefore 

 for the present merely state that they comprise : 



1. P/wsphatides, or phosphorised fats. Of these lecithin is the 

 best known ; kephalin and sphingomyelin are others. 



2. Gfalactosides ; these are nitrogenous glucosides free from 

 phosphorus ; they yield on hydrolysis the reducing sugar galactose. 



3. Cholesterin or cholesterol, a crystalline monatomic alcohol of 

 the terpene series. Its formula is C 27 H 45 OH. 



The following are some recent analyses of nerve by Falk, the 

 numbers given are percentages of the total solids : 



Medullated NOT. -med ill later! 



nerve. nerve. 



Cholesterin . . . 25 '0 47*0 



Lecithin . . . 2 '9 9-8 



Kephalin . . . 12-4 23 '7 



Galactosides . . . 18 '2 6'0 



Lecithin is a type of the phosphatides, and we may contrast its 

 decomposition products with those obtained from a fat. An 

 ordinary fat contains the elements carbon, hydrogen, and oxygen, 

 and when it takes up water it is split or hydrolysed into its con- 

 stituent parts, glycerin and fatty acid. 



Lecithin (C 42 H 84 NP0 9 ) contains not only carbon, hydrogen, and 

 oxygen, but nitrogen and phosphorus as well. When it is hydrolysed, 

 it yields not only glycerin and a fatty acid, but also phosphoric acid, 

 and a nitrogenous base termed choline. 



Lecithin + water. 



L 



Glycerin. Fatty acid Phosphoric acid. Choline. 



(usually oleic acid). 



Choline is an ammonium-like base, which contains three methyl 

 (CE 8 ) groups. Its formula is N(CH 3 ) 3 CH 2 . CH 2 (OH) 2 , and when it 

 breaks up, trimethylamine N(CH 3 ) 3 is one of its decomposition 

 products. 



Extractives. Small quantities of numerous other organic sub- 

 stances are included under this general term ; creatine, xanthine, hypo- 

 xanthine, inosite, lactic acid, uric acid, and urea have been identified. 



Inorganic salts. The proportion of mineral salts amounts to a 

 little more than 1 per cent, of the total solids. Potassium salts are 

 the most abundant. \Ve have already noted that Macdonald 

 attributes many of the phenomena of nervous action to electrolytic 

 changes in these potassium salts, though his views on that question 

 should for the present be accepted with caution (p. 164). 



Macallum uses for the micro-chemical detection of potassium an acid solution of 

 cobalt nitrite, and precipitates in situ the yellow hexanitrate of cobalt and potassium, 



