CH. XXVIII.] CHOLESTERIN 435 



protagon ; protagon may be separated into its constituents (phrenosin 

 and sphingomyelin) by pyridine, and so forth. 



The lipoids may be classified in the following way : 



(1) Those which, like the fats, are free from both nitrogen and phos- 

 phorus. The most important member of this group is cholesterin. 



(2) Those which are free from phosphorus but contain nitrogen. 

 These yield the reducing sugar called galactose when broken up, and 

 may, therefore, be called galactosides. 



(3) Those which contain both phosphorus and nitrogen. These 

 are called the phosphatides, and are grouped according to the propor- 

 tion of nitrogen and phosphorus in their molecules, as follows : 



(a) Mono - amino - mono - phosphatides, N : P = 1 : 1. E.g., 

 lecithin and kephalin. 



(&) Diamino-mono-phosphatides, N : P = 2 : 1. E.g., sphingo- 

 myelin. 



(c) Mono-amino-diphosphatides, N : P = 1 : 2. One of these, 



named cuorin, hr s been separated out from the heart by 

 Erlandsen, and a similar substance is found in egg-yolk. 



(d) Diamino-diphosphatides, N : P = 2 : 2. One of these was 



separated from brain by Thudichum, but has not since 

 been examined. 



(e) Triamino-mono -phosphatides, N: P = 3 : 1. One of these 



is present in egg-yolk. 



Cholesterin or cholesterol is found in small quantities in all 

 forms of protoplasm. It is a specially abundant constituent of 

 nervous tissues, particularly in the white substance of Schwann. 

 It is found in small quantities in the bile, but it may occur there in 

 excess and form the concretions known as gall-stones. It can be 

 readily extracted from the brain by the use of cold acetone. In the 

 brain it occurs in the free state. 



It is a monatomic unsaturated alcohol with the empirical formula 

 C 27 H 45 . OH. Eecent research has shown it to belong to the terpene 

 series, which had hitherto only been found as excretory products of 

 plant life. Windaus has shown that it contains five reduced benzene 

 rings linked together, with a double linkage at the end of an open chain. 



Cholesterin is now believed to be not merely a waste product of 

 metabolism, but to exert an important protective influence on the 

 body cells against the entrance of certain poisons called toxins. 

 One of the poisons contained in cobra venom dissolves red blood- 

 corpuscles ; the presence of cholesterin in the envelope of the 

 blood-corpuscles to some extent hinders this action, and it has 

 been stated that the administration of cholesterin increases the 

 resistance of the animal. It is certainly the case that with arti- 

 ficial blood-corpuscles, membranous bags containing haemoglobin, 



