DIGESTION 119 



occurs in the spleen, or whether this chemical change is reserved 

 for the liver. Physiologists incline to the view that the liver 

 is the seat of the change. 



Intestinal Juice. The mucous membrane of the alimentary 

 tract, as far down as the middle of the rectum, is, as previously 

 stated (p. 102), studded with tubular glands. They secrete a 

 light-yellow fluid, alkaline in reaction, and opalescent. Its 

 most important property is due to a ferment which converts 

 cane-sugar into a mixture of dextrose and levulose, and 

 changes maltose the sugar produced by the action on starch 

 oFlialiva and pancreatic juice into dextrose. It is in the 

 form of dextrose that sugar is carried about the body and 

 assimilated by the tissues. 



Intestinal juice also contains a ferment, erepsin, which shakes 

 to pieces the heavy molecules of peptones and partly formed 

 peptones. Under its influence they break up into compara- 

 tively simple bodies containing the radicle of ammonia. 

 Substances containing an NH 2 group one H of NH 3 (ammonia) 

 having been given up, in order that the group may have a 

 " free arm " with which to link on to the other component parts 

 of the molecule are termed " amides." The amides which 

 are most characteristic of the action of erepsin are leucin, an 

 amidated fatty acid ; and tyrosin, an amidated aromatic acid. 

 The tendency of proteins to break up along these two lines 

 the fatty acid line and the aromatic acid line is of consider- 

 able interest. The one line is represented by acetic acid, 

 CH 3 ,COOH ; the other contains the hexone radicle, C 6 H 6 . Ben- 

 zoic acid, C 6 H 5 COOH, is representative of the latter. It used 

 to be thought that proteins which were shaken into simple bodies 

 such as amides were lost to the economy. Their downward 

 career was a foregone conclusion. There could be no arresting 

 it before they brought up at the bottom as urea, CO(NH 2 ) 2 

 the diamide of carbonic acid. It was even supposed that this 

 disintegration of proteins was a provision for getting rid of 

 the surplus animal food which we consume. Physiological 

 chemists now take quite a different view. They believe that 

 the epithelial wall of the intestine through which these sub- 

 stances are absorbed, or the liver, to which they are carried 

 by the portal blood-stream, has the power of recombining 

 these fragments into the complex protein edifice. It is even 



