CHEMICAL BASIS OF THE ANIMAL BODY. 1237 



on polarized light (a) D =— 3-5 in ethereal solution, = — 37' 

 in chloroform ic. 



Cholesterin occurs in small quantities in the blood and 

 many tissues, and is present in abundance in the white matter 

 of the cerebro-spinal axis and in nerves. It is a constant con- 

 stituent of bile, and forms frequently nearly the whole mass of 

 some gall-stones. It is found in many pathological fluids, 

 hydrocele, the fluid of ovarial cysts, etc., also in fasces and milk. 

 It also occurs in the substance of the crystalline lens, more 

 especially in 'cataract.' 



Fig. 199. Cholesterin Crystals. (After Funke.) 



Preparation. Gall-stones supply the most convenient source of 

 cholesterin. These are pounded, extracted with boiling water and 

 dissolved in boiling alcohol. The solution is filtered through a 

 heated filter, and the cholesterin separates out in a fairly pure con- 

 dition as the filtrate cools. It is purified by resolution in boiling 

 alcohol to which some caustic soda has been added, from this it 

 again separates on cooling, and is finally washed with cold alcohol 

 and water. 



Cholesterin is characterized, apart from its crystalline form, 

 by some striking reactions which may be obtained even with 

 microscopic quantities. 



(i) When the crystals are treated with concentrated 

 sulphuric acid they usually turn violet or red. On the addi- 

 tion of a little iodine the play of colours is very marked, the 

 crystals being variously coloured, — blue, red, green, violet. 



(ii) When dissolved in chloroform, the solution turns 

 blood-red on the addition of an equal volume of concentrated 

 sulphuric acid: this turns to blue, green and finally yellow, 

 the change of colour being veiy rapid if the solution is freely 

 exposed to the air in an open dish. The sulphuric acid under 

 the chloroform exhibits a green fluorescence. 



