504 DIGESTION. 



ration. The crystals must be purified as above directed. 

 ,07? S . (1) Put a few crystals of cholesterin under the 

 microscope ; add a drop of a mixture of five volumes of sul- 

 phuric acid and one of water, and warm the object-glass gently. 

 The edges of the crystals will acquire a carmine color. If 

 three parts of acid are used to one of water, the edges are 

 violet, and if it is still more dilute they become lilac and dis- 

 solve in the acid. (2) Add to some crystals strong sulphuric 

 acid, with a little iodine or zinc chloride ; they acquire a tint 

 which varies from greenish-blue to violet. (3) Put a drop of 

 roneeut rated nitric arid on a crystal in a porcelain capsule, 

 and evaporate to dry ness at a gentle heat ; touch the residue 

 with a drop of ammonia. A deep red color is produced. (4) 

 Hub up cholesterin with strong sulphuric acid, and add chlo- 

 roform. A solution varying in color from blood-red to purple 

 is produced, which, after changing successively into violet, 

 blue, and green, finally disappears. 



* 148. Action of Bile. The bile appears to aid the ab- 

 sorption of fat. Lenz, Bidder, and Schmidt found that, after 

 ligature of the gall duct, a dog absorbed less fat than before, 

 and that the chyle* in the thoracic duct contained very little 

 fat. They calculated the amount absorbed by comparing the 

 quantity of fat eaten with the amount passed with the faeces. 

 The bile emulsionizes fat, as can be seen by shaking a little oil 

 with it. It is doubtful, however, whether it is to this property 

 that the absorption is due. In forcing oil through animal mem- 

 branes or filter-paper, either by pressure or by suction, it passes 

 with much greater facility if it has been previously mixed with 

 bile. 



149. Bile precipitates Syntonin and Pepsin. Digest 

 a piece of fibrin with artificial gastric juice, and then add a 

 larijv quantity of bile to it; a precipitate is at once produced. 

 Filter, put another piece of fibrin in the filtrate, and acidulate 

 with hydrochloric acid to the proper degree. The pepsin having 

 been precipitated) the fibrin is not digested. Unless the quan- 

 tity of bile is large, the whole of the pepsin will not be thrown 

 down. It is not known what purpose is served by the pre- 

 cipitation of the chyme by the bile in the duodenum. In the 

 stomach the presence of bile must be injurious. 



150. Secretion of Bile. The secretion of bile goes on 

 constantly, but is more rapid at one time than another. It is 

 accelerated after taking food, usually attaining its maximum 

 from two to four hours after each meal. The secretion is 

 observed by tying the gall duct and introducing a canula into 

 the gall bladder. A detailed account of the method of perform- 

 in^ this operation on dogs is given by Rutherford and Gamgee in 

 the report of the British Association for 1868. The principal 

 facts may be demonstrated in the guineapig, as follows : 



