DEFINITION OF "INTERNAL SECRETION" 13 



to the stage of activity of the gland. It is not, perhaps, 

 possible to insist on the recognition of granules as definite as 

 those in the pancreatic cells before we admit a structure into 

 the category of internally secreting glands, but it is essential 

 that the constituent cells should have the general character 

 of glandular i.e., secretory cells. This is in some instances 

 not altogether an easy matter to determine, and, as we shall 

 see later on, there is still some discussion as to whether such a 

 tissue as the chromaphil may reasonably be supposed to have 

 a secretory function. That a discussion of this kind should 

 arise in connection with a structure generally supposed to be 

 internally secretory shows how little we know about the actual 

 act of secretion in such a case (see p. 216 et ff.). 



We conclude, then, that secretion (internal or external) 

 represents a highly specialized grade of metabolic activity, 

 and should be distinguished as rigorously from general meta- 

 bolism as the contraction of muscle from general motility 

 (Kohn). 



Kohn gives a very excellent illustration of the two processes, 

 external secretion and internal secretion. The manufacture 

 of the bile and its conveyance into the duodenum is a secretion 

 in the ordinary sense of the word an " external" secretion. 

 When we obstruct the bile ducts, the secretion goes on just 

 the same ; but now the bile is conducted into the blood-stream, 

 and we get an " internal secretion " of bile. This shows that 

 the products of secretion can, under certain circumstances, 

 pass into the circulation. And we can be tolerably certain 

 that this process can in some tissues occur normally. As we 

 have seen, it is sometimes difficult to decide whether a given 

 tissue is glandular or not, and therefore whether we ought to 

 ascribe to it a secretory function. Thus, Kohn admits the 

 "cortex" only of the adrenal among the glands, while he 

 insists that the " medulla " consists of " chromaphil cells," 

 which are not secretory, not epithelial, and therefore cannot 

 secrete. This point will be referred to later and more fully 

 under the head of the adrenal body (see p. 216 et ff.). It may 

 be well, however, to remark in passing that it is from the 

 medulla, and not the cortex, that the active principle is 

 obtained. 



We are now in a position to define internal secretion. The 

 process consists in the preparation and setting freq of certain. 



