Chap. IX.] NUTRITION AND METABOLISM. 185 



trypsin. It will also (3) curdle milk through a rennet ferment, 

 And lastly (4) it has an important action on fatty and oily sub- 

 stances. Not only does it (a.) convert them into an emulsion, 

 that is, cause them to be suspended in very minute globules, like 

 the butter globules in milk, as may be seen by shaking up some 

 oil with the extract, but it also, (b.) not improbably through the 

 action of a fourth ferment (steapsin), splits up the fats into their 

 fatty acids and glycerine. These fatty acids, with an alkaline 

 carbonate, of which there is about -75 per cent, in pancreatic 

 juice, form soluble soaps, and these soaps further aid in emulsi- 

 fying the fats. Pancreatic juice, therefore, (1) converts starch 

 into sugar; (2) converts proteids into peptones; (3) coagulates 

 casein from milk ; and (4) emulsifies and saponifies fats. 



The product, or rather a product, of the liver is bile. This is 

 a viscid, slightly alkaline fluid, green in the rabbit and most 

 herbivorous animals, reddish in the carnivora. It contains (1) 

 certain organic bile salts (sodium glycocholate and taurocholate) ; 

 (2) certain organic colouring matters ; and (3) fatty matter and 

 cholesterin (an univalent alcohol), together with mucous and 

 inorganic salts. Its main functions in digestion would seem 

 to be (1) emulsifying fatty matters; and (2) precipitating the 

 gastric peptones, together with pepsin and bile acids. In 

 addition to these functions it assists digestion by moistening the 

 mucous membrane and so facilitating absorption, and by acting 

 as a natural purgative. 



Like the other secretions before considered, the bile is not 

 derived as such from the blood, but is elaborated out of the 

 materials of the blood by a process of katabolism, and it is 

 possible that there may be hitherto undiscovered katastates. 

 The process of secretion goes on most rapidly during digestion : 

 but it does not cease at other times. At such times the bile, 

 instead of passing down the common bile-duct into the duodenum, 

 regurgitates through the cystic duct into the gall-bladder, where 

 it is stored up for future use. 



Having thus considered the general structure of the glands, 

 the nature of their products, and the part they play in digestion, 

 we may now pass on to consider the process of digestion 



