THE DIGESTIVE JUICES 



93 



FIG. 25. Cholesterin crystals. 



in the region of the E line. Hydrobilirubin differs from urobilin in 

 containing much more nitrogen in its molecule (9*2 instead of 4*1 per 

 cent.), and is probably a product of less complete reduction than 

 urobilin. (See further Lesson XXVI.) Urobilin is also formed, by 

 the oxidation of haemopyrrol (see Haemoglobin, p. 114). 



Cholesterin. This substance is contained, not only in bile, but 

 very largely in nervous tissues. Like lecithin, it is an abundant 

 constituent of the white substance of 

 Schwann. It is found also in blood cor- 

 puscles, and in blood plasma as an ester 

 of oleic and palmitic acids. The cholesterin 

 of nervous tissues is, however, free. In 

 bile it is normally present in small quanti- 

 ties only, but it may occur in excess, and so 

 form the concretions known as gallstones, 

 which are generally more or less tinged 

 with bilirubin. 



Though its solubilities remind one of a 

 fat, cholesterin is not a fat, but a monatomic 

 alcohol, probably of the terpene series. Its formula is C 2 7H 45 .HO. 



From alcohol or ether containing water it crystallises in the form 

 of rhombic tables, which contain a molecule of water of crystallisation : 

 these are easily recognised under the microscope (fig. 25). It gives 

 the tests described under the practical exercises on p. 79. What the 

 physiological uses of cholesterin are is entirely unknown. 



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. 



THE USES OF BILE 



Bile is doubtless, to a certain extent, excretory. In some animals 

 it has a slight action on fats and starch, 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. Its auxiliary action in 

 starch digestion has been shown in one of our practical exercises 

 (p. 78). It has a similar assisting power in the digestion of 

 proteins. 



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

 processes in the intestine. This is 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 



