GLANDS, GENERAL PHYSIOLOGY OF SECRETION. 255 



things never possible in simple osmosis. Fourth, glands 

 may be made to secrete even when they contain no circu- 

 lating blood. Thus, if the sciatic nerve in an amputated 

 limb of an animal be stimulated the sweat glands may be 

 made to secrete, when it is evident that there is no circu- 

 lating blood in the severed limb. We are driven, therefore, 

 to the conclusion that secretion is a phenomenon of the liv- 

 ing gland cells themselves, and its ultimate nature we un- 

 derstand as little as we do the ultimate nature of muscle 

 activity or nervous impulses. It is a chemical process whose 

 explanation is not yet at hand. 



3. Histological Changes in Secreting Cells. On the 

 other hand, while we do not understand the exact nature of 

 the process of secretion, we are able to observe on a gland 

 certain histological changes in rest and in action. Thus, 

 when a gland starts to secrete it at once becomes flushed 

 with blood, due to the enlargement of the arteries traversing 

 it. Such a dilatation has, however, nothing to do with the 

 gland itself, but has been brought about by nerves which run 

 to the arteries direct, and which were stimulated at the 

 same time the gland cells were stimulated. The purpose of 

 such an increase in the supply of blood to a working gland 

 is, of course, very apparent. It is to carry abundant ma- 

 terial to the gland, and supply it with sufficient amounts of 

 oxygen to sustain its activities. It differs in no essential 

 way from the condition of things in a working muscle. 



But histological changes may be observed in the secret- 

 ing cells themselves, for a gland that has been actively 

 secreting looks quite different under the microscope from 

 one which has been at rest for some time. If, for instance, 

 two animals as nearly alike as possible be taken, and one 

 of them be starved for a day, and thus the pancreas of that 

 animal be given no occasion to pour out its secretion, and 

 the animal be then killed and the pancreas observed histo- 

 logically, it may be seen that the gland cells are distended 

 and that their outer portions, that is, the portions next to 

 the lumen of the gland, are filled with granules, while those 



