74 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



1. With iodine and concentrated sulphuric acid the crystals give 

 a play of red, green, and blue. 



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

 crystals turn red. 



3. A solution of cholesterin in chloroform, gently shaken with an 

 equal amount of concentrated sulphuric acid, turns red, and ultimately 

 purple, the subjacent acid acquiring a green fluorescence. (Salkow- 

 ski's reaction.) 



' The mode of origin of cholesterin in the body has not been clearly 

 ma'de out. Whether it is formed in the tissues generally, in the 

 blood, or in the liver, is not known ; nor has it been determined con- 

 clusively that it is derived from albuminous or nervous matter. It is 

 also doubtful if we are to regard it as a waste substance of no use to 

 the body, as its presence in the blood-corpuscles, in nervous matter, 

 in the egg, and in vegetable grains, points to a possible function of a 

 histogenetic or tissue-forming character.' (McKendrick.) 



A substance called iso-cholesterin, isomeric with ordinary chole- 

 sterin, 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 does not give Salkowski's reaction with chloroform and sul- 

 phuric acid just described. 



THE USES OF BILE 



One of the most remarkable facts concerning the bile is its 

 apparently small use in the digestion of food. It is doubtless, to a 

 large extent, excretory. Some state that in some animals it has a slight 

 action on fats and carbohydrates, but it appears to be rather a coadjutor 

 to the pancreatic juice (especially in the digestion of fat) than to have 

 any independent digestive activity (see p. 64). 



Bile is said to be a natural antiseptic, lessening the putrefactive 

 processes in the intestine. This is also very doubtful. Though the 

 bile salts are weak antiseptics, the bile itself is readily putrescible, 

 and the power it has of diminishing putrescence in the intestine is due 

 chiefly to the fact that by increasing absorption it lessens the amount 

 of putrescible matter in the bowel. 



When the bile meets the chyme the turbidity of the latter is in- 

 creased, owing to the precipitation of unpeptonised proteid. This is an 

 action due to the bile salts, and it has been surmised that this con- 

 version of the chyme into a more viscid mass is to hinder somewhat 

 its progress through the intestines ; it clings to the intestinal wall, 

 thus allowing absorption to take place. The neutralisation of the acid 

 gastric juice by the bile also allows the alkalinity of the pancreatic 



