PHYSIOLOGY OF THE DIGESTIVE GLANDS 429 



The copious secretion of pancreatic juice produced by secretin, on the 

 one hand, and the scanty, thick secretion produced by vagus stimula- 

 tion, on the other, calls to mind similar differences observed in the secre- 

 tion of saliva as the result of chorda-tympani or sympathetic stimulation. 

 It will be remembered that from these latter results it was concluded 

 that there must be secretory and trophic fibers concerned in the control 

 of the activities of gland cells. Interesting corroboration of this conclusion 

 has recently been obtained by histological examination of the pancreas fol- 

 lowing secretin or vagus activity. After the repeated injection of secre- 

 tin, it is difficult to observe any signs of fatigue in the cells ; the zymogen 

 granules remain practically as numerous as in a resting gland, but in the 

 clear protoplasm of the outer third of the cell, it is said that fine channels 

 of fluid can be seen. Through these channels water is believed to pass 

 from the blood towards the lumen and, in its course to carry with it some 

 of the zymogen granules, without, however, changing them. Thus, when 

 the gland cells are stained with eosin and orange, after secretin activity 

 some of the zymogen granules can occasionally be seen in the lumen of 

 the acini stained with eosin like those in the cell itself. After vagus 

 stimulation the appearances are different ; not only are the granules more 

 freely extruded from the cells, but they undergo a preliminary change; 

 they lose the property of staining with eosin and become stained with 

 orange, at the same time increasing in size so as to form vacuoles. 

 These vacuoles may wander into the ductules, and when they are present 

 here they are stained by orange (Figs. 146 and 147) (Babkin, etc. 7a ). 



Why there should be both a nervous and a hormone control of the pan- 

 creatic secretion is not clear. This gland, unlike the gastric and salivary 

 glands, is not called upon to become active all of a sudden, and it is dif- 

 ficult to see what could serve as the normal stimulus operating through 

 the nervous pathway. Taking it all in all, it is probably safe to con- 

 clude that the nervous mechanism is relatively unimportant, and that 

 under normal conditions it seldom if ever is called into operation. Cor- 

 roboration for this view is afforded by the fact, above mentioned, that 

 the pancreatic juice produced by vagus stimulation contains active tryp- 

 sin, which is not the case with normal pancreatic juice. 



