166 MANUAL OF PHYSIOLOGY. 



duction of food into the stomach, is certainly brought about by 

 nervous agencies of a similar nature to that of the stomach, the 

 secretion of which follows closely upon mastication. The second 

 accompanies the passage of the undigested food through the 

 small intestines, and may also be most conveniently explained 

 as the result of reflex nervous stimulation of the gland cells. 



The great complexity of the nerve distribution to the glands of 

 the intestinal tract makes it difficult to ascertain the exact chan- 

 nels traversed by the afferent and efferent impulses. The follow- 

 ing observations, if accurate, would tend to prove that certain 

 inhibitory impulses pass from the stomach along the vagus to the 

 medulla, and are thence reflected to the gland by its vasomotor 

 nerves. During vomiting, or when the central end of the divided 

 vagus is stimulated, the secretion of the pancreas ceases. Section 

 of the nerves which surround the blood vessels distributed to the 

 pancreas causes considerable (paralytic) flow of secretion which 

 stimulation of the vagus cannot check. 



No nerve channels have been demonstrated to carry exciting 

 impulses direct to the glands, as the chorda tympani does to the 

 sub-maxillary ; but the direct stimulation of the gland itself, or 

 of the medulla oblongata, is said to induce activity of the gland. 



During the period of rest of the pancreas, i. e., when the ali- 

 mentary tract is not in activity, no secretion flowing from the duct 

 and the gland being pale, the gland cells in the acini undergo a 

 change which may be compared with that observed in the cells 

 of the serous salivary glands. The division of the row of cells 

 lining the acinus, into a central glandular and outer clear zone, 

 has already been mentioned. 



Immediately after very active secretion, the central granular 

 zone is reduced to a minimum owing to the paucity of granules ; 

 and the outer zone occupies the greater part of the cell, the entire 

 substance of which stains readily and looks like ordinary proto- 

 plasm. After rest, however, the granules reappear, and after the 

 lapse of a short, quiescent period, the inner granular zone has 

 again encroached on the outer, owing to the accumulation of gran- 

 ules which, rapidly increasing, fill the greater part of the cells, 

 and cause them to bulge inward and occlude the lumen of the 



