82 THE WORK OF THE DIGESTIVE GLANDS. 



the pancreas and by the digestive glands during the course of the 

 same act of digestion does not remain uniform, but is varied in many 

 respects. It is in the highest degree both interesting and important 

 to determine in what way these variations are related to the progress of 

 the act of digestion, and whether they are of service to it. A complete 

 solution of this problem must be left to the future. Some of the 

 details, however, disclose an obvious purpose. Take, for example, the 

 first secreted portions of gastric juice : they are distinguished from the 

 others by a stronger digestive power. It is evident that at the begin- 

 ning of digestion, when the quantity of food is large and its external 

 structure still coarse, this is well timed. The strongest juice is thus 

 poured out when it is most needed. 



In the case of the pancreatic juice it is much more difficult to show 

 that the alterations in its composition are purposive. Here the matter 

 concerns a later stage in the work of our factory, where a food material 

 already modified and assorted by the stomach has to be further worked 

 up. To this end chemical conditions must be provided in the intestine 

 which help on the action of the pancreatic juice. This means that the 

 conditions under which gastric digestion has been accomplished must be 

 radically changed, since they are injurious to the action of the pancreas. 

 We know that trypsin is digested by pepsin, and that a high degree of 

 acidity injuriously influences its activity. I merely raise these questions 

 now. Their elucidation will be taken in hand after we have discussed 

 the mechanism of excitation of the glands. 



The facts already communicated indicate that the glands are able to 

 adapt themselves to the separate and successively occurring phases of 

 the elaboration of the food. In connection therewith we may rightly 

 suppose that this adaptability will only appear to its full extent when 

 we compare the variations in the progress of secretion under different 

 dietaries with each other. Since the food is made up of several con- 

 stituents, and since different juices are poured out into the alimentary 

 canal, the supposition appears natural that each fluid, with accentuation 

 of certain of its properties, is furnished chiefly for a particular kind of 

 food. 



Is this in reality the case ? It is obvious that an answer to this 

 question was impossible by the aid of the older methods. Now, how- 

 ever, the fact that we are able to consider it is, of itself, a brilliant 

 testimony to the essential service rendered by our newer methods. At 

 present we can convince ourselves by actual experiment of what a priori 

 only appeai-ed probable, viz., that every individual kind of food calls 

 forth a particular activity of the digestive glands, with special pi-o- 

 perties of the digestive juices. 



We may commence with the stomach. Researches carried out by 



