CH. XXYIII.] CHOLESTERIN 437 



be lipoids in the fluid crystalline condition. Pure cholesterin and 

 pure cholesterin esters do not exhibit the phenomenon ; but mixtures 

 of cholesterin and fatty acids do; it has been suggested that in 

 such mixtures the acid is incorporated as "acid of crystallisation," 

 analogous to the " water of crystallisation " in many other crystals. 



The Galactosides. The substance known as protagon can be 

 separated out from the brain by means of warm alcohol ; on cooling 

 the extract, protagon is deposited as a white precipitate. This, how- 

 ever, also contains cholesterin, which can be dissolved out by ether. 

 Another method of preparing protagon is to take brain and extract 

 the cholesterin first with cold acetone ; then hot acetone is employed 

 to extract the protagon. Protagon is a substance originally described 

 by Couerbe, under the name cerebrote, but named protagon by 

 Liebreich, who regarded it as a definite compound, and the mother 

 substance of all the other phosphorised and non-phosphorised con- 

 stituents of the brain. It has now been definitely proved in confir- 

 mation of what Thudichum stated in 1874, that protagon is not 

 important quantitatively, and is not a definite chemical unit, but a 

 mixture of phosphorised and non-phosphorised substances in such 

 proportions that it usually contains about 1 per cent, of phosphorus. 

 By treatment with appropriate reagents and recrystalhsation, pro- 

 tagon can be separated into its constituents, and those which are free 

 from phosphorus and comprise about 70 per cent, of the original 

 protagon are the galactosides. The known galactosides are two in 

 number, namely, phrenosin (or cerebron) and kerasin. The former 

 is a crystalline product, and the latter of somewhat waxy con- 

 sistency. They are probably isomerides. They yield on decomposi- 

 tion three substances : (1) A reducing sugar, galactose. (2) A base 

 termed sphingosine, about which little chemically is yet known. 

 (3) A fatty acid of high molecular weight, termed neuro-stearic 

 acid by Thudichum, but not definitely identified. It is probably an 

 oxy-acid (Thierfelder). 



The Phosphatides. The best known of these is lecithin. 

 This is a very labile substance, but it yields on decomposition four 

 materials, namely glycerin and phosphoric acid united together as 

 glycero-phosphoric acid, two fatty acid radicals, of which one is 

 usually oleic acid, and an ammonium-like base termed choline. The 

 fatty acid radicals are united to glycerin as in an ordinary fat, the 

 place of the third fatty acid being taken by the radical of phosphoric 

 acid, which in its turn is united in an ester-like manner to the 

 choline. The clinical significance of such substances in cases of 

 degenerative nervous disease has been already alluded to on p. 167. 



Kephalin resembles lecithin in being a mono-amino-monophos- 

 phatide. It differs from lecithin in being insoluble in alcohol. On 

 decomposition it yields glycero-phosphoric acid, certain fatty acids 



