436 



THE CHEMICAL COMPOSITION OF THE BODY [CH. XXV11I. 



FIG. 315. Cholesterin crystals. 



the impregnation of the membrane with cholesterin, prevents the 



solvent action of toxins. 



In order that cholesterin and its derivatives may act in this way, it 



is necessary that the double linkage and 

 the hydroxyl atom just referred to 

 should be intact. The latter would 

 not be the case in an ester, and it is 

 probable that the compounds of choles- 

 terin in the blood previously described 

 as esters by Hiirthle are really mixtures 

 of cholesterin and fatty acids. 



From alcohol or ether containing 

 water it crystallises in the form of rhom- 

 bic tables, which contain one molecule of 

 water of crystallisation : these are easily 

 recognised under the microscope (fig. 315). 

 It gives the following colour tests : 



1. Heated with sulphuric acid and water (5:1), the edges of the 

 crystals turn red. 



2. A solution of cholesterin in chloroform, shaken with an equal 

 amount of strong sulphuric acid, turns red, and then purple, the 

 subjacent acid acquiring a green fluorescence. (Salkowski's reaction.) 



3. If acetic anhydride is added to a chloroformic solution of 

 cholesterin, and then sulphuric acid, drop by drop, a red coloration, 

 which changes to bluish green, is produced. (Liebermann's reaction.) 



A substance called iso-cholesterin is found in the fatty secretion 

 of the skin (sebum) ; it is largely contained in the preparation called 

 lanoline, made from sheep's wool fat. It differs from cholesterin in 

 being dextro-rotatory instead of Isevo-rotatory in solution, and it 

 does not give Salkowski's colour reaction. Cholesterins isomeric with 

 animal cholesterin are also found in many plants ; these are termed 

 phyto-cholesterins, or phytosterins for short. 



Cholesterin compounds exhibit the physical phenomenon recently 

 studied by Lehmann, namely, the formation of liquid crystals ; 

 this is also shown by several other lipoids. Virchow in 1855 

 described what he termed " myelin forms " ; if brain-substance 

 is mixed with water, where the water touches the brain material, 

 threads are observable shooting out and twisting into fantastic 

 shapes ; these are termed " myelin forms/' although the word myelin 

 has no definite chemical meaning. It has now been shown that these 

 " myelin forms " are distorted liquid crystals due to the presence of 

 cholesterin and other lipoids. The fat globules seen in the adrenal 

 cortex, and in the liver and other organs during fatty degeneration, 

 are not wholly composed of fat, for the polarisation microscope shows 

 them to be anisotropic, and further investigation has shown them to 



