DIGESTION 



383 



stance which we may call pro-trypsinogen, since it is supposed to 

 be carried in the blood to the pancreatic cells, and changed by 

 them into trypsinogen. There is some evidence that extracts 

 of the spleen prepared from it when congested during digestion 

 exert a favourable influence on the proteolytic power of the 

 pancreas (Mendel). And there is no doubt that the spleen, like 

 other organs, contains an intracellular enzyme which can aid 

 in the digestion of protein. The products of the action in an 

 acid medium of this enzyme are the same as those formed by 

 trypsin in an alkaline medium 

 (Leathes). But this is not enough 

 to prove that the spleen has any 

 special relation to pancreatic diges- 

 tion. 



The Influence of Nerves on the 

 Secretion of Bile. Although bile is 

 secreted constantly, it only passes at 

 intervals into the intestine. For the 

 liver in many animals, unlike every 

 other gland except the kidney, has 

 in connection with it a reservoir, 

 the gall-bladder, in which its secre- 

 tion accumulates, and from which 

 it is only expelled occasionally. We 

 have therefore to distinguish the 

 bile-secreting from the bile-expelling 

 mechanism, To study the rate of 

 secretion of bile (Fig. 151), a fistula 

 of the gall-bladder can be established. 

 But to learn the function of bile in 

 digestion it is more important to 

 know when and at what rate it enters 

 the intestine. For this purpose a 

 fistula is made by cutting the natural 

 orifice of the common bile-duct with 



a piece of the surrounding mucous membrane out of the intestine 

 and transplanting it upon the serous coat, where it is sutured. The 

 loop of intestine, with the orifice of the duct facing outwards, is 

 then stitched into the abdominal wound, where it is allowed to 

 heal. Of course, since a circulation of the bile-acids takes place 

 i.e., an absorption from and re-excretion into the intestine 

 the formation of that juice cannot proceed upon absolutely 

 normal lines when the bile no longer enters the duodenum. 

 The only condition under which fistula bile could have the same 

 composition as normal bile would be that in which as great an 

 amount of bile-acids is introduced into the gut as escapes through 



FIG. 151. RATE OF SECRETION 

 OF BILE. 



S shows how the rate of secre- 

 tion of bile falls in a dog when a 

 biliary fistula is first made, and 

 the bile thus prevented from 

 entering the intestine ; P shows 

 the fall of the percentage of 

 solids. The numbers along the 

 horizontal axis are quarters of 

 an hour since bile began to escape 

 through the fistula. The num- 

 bers along the vertical axis refer 

 only to curve S, and represent 

 the rate of secretion in arbitrary 

 units. 



