58 A TEXTBOOK OF PHYSIOLOGY 



called because upon hydrolysis they yield the reducing sugar galactose. 

 In addition they also yield a base known as sphingosin, and an acid 

 of a fatty acid nature. The two chief members of this group are 

 cerebrin, or phrenosin, and kerasin. 



III. THE CHOLESTEROLS. Cholesterol, C^H^OH, chemically speak- 

 ing belongs to quite a different series of bodies. It contains neither 

 nitrogen nor phosphorus, and is probably a monatomic alcohol of the 

 terpene series. Bodies of this series are common in plants, examples 

 being camphor and turpentine. The following formula has been 

 suggested : 



(CH 3 ) 2 CM CH 2 CH 2 C 17 H 2 (5 CH = CH 2 



CM, CrL> 

 CH(OH) 



It occurs in all the cells and fluids of the body in small quantities, 

 but particularly in nervous tissue and in the envelope of the red blood- 

 corpuscles. It is also present in the bile, from which in catarrhal 

 states of the bile-ducts it may sometimes separate, forming biliary calculi. 



Cholesterol is probably not merely a waste product as it was once 

 deemed to be. As stated above, it hinders the action of ferments and 

 also that of certain poisons, such as snake venom and saponin, upon 

 the red blood-corpuscles. 



It is sometimes found as an ester in the liver and plasma in com- 

 bination with a fatty acid such as oleic, palmitic, or stearic. It occurs 

 also in the suprarenal capsules in this form. 



Cholesterol is soluble in ether, chloroform, and warm alcohol. Its 

 solutions are dextrorotatory. From these it crystallizes in charac- 

 teristic colourless transparent plates, each with a small piece knocked 

 out of the corner. It is insoluble in water, dilute acids, or alkalies. 

 The most general tests for cholesterol are 



1. A red colour results when concentrated sulphuric acid is 

 added to a chloroform solution of cholesterol. (Salkowski's test.) 



2. A play of colours, red, blue, green, is obtained when the 

 same acid is added to a solution of cholesterol in acetic anhydride. 

 (Liebermann's test.) 



Isocholesterol, a body similar to cholesterol in most respects, is 

 found in sebum. It differs from cholesterol in not giving Salkowski's 

 test, and in being levorotatory in solution. 



Phytocholesterol is the special name applied to the cholesterols 

 of plants, from which animal cholesterols are probably derived. 



Cephalin is found largely in both the white and grey matter of 

 nervous tissues. It is soluble in ether and chloroform, but not in alcohol. 



Sphingomyelin is found in protagon, and also in the suprarenal 

 capsules. 



Jecorin is found in the liver. It is probably a combination of a 

 phosphatide and a sugar. 



