382 A MANUAL OF PHYSIOLOGY 



are related to the nature of the food, then the amount of secretin 

 formed, and therefore the intensity of secretion in the pancreas, 

 will be similarly related. The one apparently proved example 

 of specific adaptation of the pancreatic juice has not stood 

 the test of a critical examination. It was asserted that in 

 dogs fed for some days with food containing lactose (milk) the 

 ferment, lactase, is present in that secretion, while the pancreatic 

 juice of dogs whose food is free from lactose does not contain 

 lactase. The adaptation of the pancreas to lactose was sup- 

 posed to be achieved through some substance produced by the 

 action of lactose on the intestinal mucous membrane, which 

 plays the part of a specific chemical stimulus to the pancreatic 

 cells or their secretory nervous mechanism, causing them to form 

 lactase. But it has been conclusively shown that when dogs are 

 fed with lactose for weeks no lactase appears in the pancreatic 

 juice (Plimmer). 



The natural secretion of pancreatic juice is by no means so 

 intermittent as that of saliva. In the rabbit the pancreatic, like 

 the gastric, juice flows continuously. In the dog it begins almost 

 as soon as food is taken, rises in two or three hours to a maximum, 

 then falls till the fifth or sixth hour, after which it may mount 

 again somewhat, and then, gradually diminishing, ultimately 

 stops (Figs. 149, 150). During normal activity the blood- 

 vessels of the gland are dilated. But under experimental condi- 

 tions the increased secretion caused by secretin is accompanied 

 sometimes by an increase and sometimes by a diminution in the 

 blood-flow, and secretion may continue for some time after com- 

 plete cessation of the circulation, while the increased consumption 

 of oxygen which goes hand in hand with the increased secretion is 

 also independent of the blood-supply (May, Barcrof t and Starling) . 

 This shows how far the secretory process is from a mere mechanical 

 filtration, although it does not follow that, under normal condi- 

 tions, 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 gands. (Edema begins 

 before a manometer in the duct shows a pressure of 20 mm. of mer- 

 cury, 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 important role in the elaboration 

 of the proteolytic ferment of the pancreas, forming a sub- 



