INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 405 



while the increased consumption of oxygen which goes hand in hand 

 with the increased secretion is also independent of the blood-supply 

 (May, Barcroft and Starling) . This shows how far the secretory pro- 

 cess is from a mere mechanical filtration, although it does not follow 

 that, under normal conditions, a decreased blood-flow ever does 

 accompany an increased secretion. There is one difference between 

 the normal secretion of pancreatic juice and of saliva which may still 

 be mentioned : the pressure of the latter in the submaxillary duct may, 

 as we have seen, greatly exceed the arterial blood-pressure, without 

 reabsorption and consequent oedema of the gland occurring ; but the 

 secretory pressure of the pancreatic cells is very low, not more than 

 a tenth of that of the salivary glands. 

 (Edema begins before a manometer in 

 the duct shows a pressure of 20 mm. 

 of mercury, the secreted fluid passing 

 very easily into the lymph spaces. 



The mutual relations of the spleen 

 and pancreas have formed the subject 

 of numerous inquiries. Some authors 

 maintain that the spleen plays an im- 

 portant role in the elaboration of the 

 proteolytic ferment of the pancreas, 

 forming a substance 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 trypsin- 

 ogen. 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 digestion. 



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 secretion accumulates, and from which it is only expelled 

 occasionally. We have therefore to distinguish the bile- secretion 

 from the bile-expelling mechanism. To study the rate of secreting 

 of bile (Fig. 168), a fistula of the gall-bladder can be established. 



Fig. 168. Rate of Secretion of 

 Bile. S shows how the rate of 

 secretion of bile falls in a dog 

 when a biliary fistula is first 

 made, and the bile thus pre- 

 vented from entering the intes- 

 tine ; P shows the fall of the per- 

 centage of solids. The numbers 

 along the horizontal axis are 

 quarters of an hour since bile 

 began to escape through the 

 fistula. The numbers along 

 the vertical axis refer only to 

 curve S, and represent the rate 

 of secretion in arbitrary units. 



