THE FOOD AND DIGESTION 341 



The centre which presides over the act is in the medulla 

 oblongata, and while it is usually reflexly called into action, 

 it may be stimulated directly by such drugs as apomorphine. 



IY. Intestinal Digestion 



After being subjected to gastric digestion the food is gene- 

 rally reduced to a semi-fluid grey pultaceous condition of 

 strongly acid reaction known as chyme, and in this condition 

 it enters the duodenum. 



Here it meets three different secretions : 

 Bile. 



Pancreatic secretion. 

 Intestinal secretion. 



A. Bile. 



i. Characters and Composition. The bile is the secretion 

 of the liver, and it may be procured for examination (a) 

 From the gall bladder, or (6) from the bile passages by 

 making a fistula into them. Bile which has been in the 

 gall bladder is richer in solids than bile taken directly from 

 the ducts, because water is absorbed by the walls of the 

 bladder and the bile thus becomes concentrated. 



Analyses of gall bladder bile thus give no information as 

 to the composition of the bile when formed. In several cases, 

 where surgeons have produced biliary fistulse, opportunities 

 have occurred of procuring the bile directly from the ducts 

 during life in man. 



Such bile has a somewhat orange-brown colour, and is 

 more or less viscous, but not nearly so viscous as bile taken 

 from the gall bladder. It has a specific gravity of almost 

 1005, while gall bladder bile has a specific gravity of about 

 1030. Its reaction is slightly alkaline, and it has a charac- 

 teristic smell. 



It contains about 2 per cent, of solids, of which more than 

 half are organic. 



The most abundant solids are the salts of the bile acids. 

 In man the most important is glycocholate of soda. Tauro- 

 cholate of soda occurs in small amounts. These salts are 

 readily separated from an alcoholic solution of dried bile by 



