158 HUMAN PHYSIOLOGY. 



Mechanism of Secretion. During the time of lactation the mam- 

 mary gland exhibits periods of secretory activity which alternate 

 with periods of rest. Coincidentally with these periods, certain his- 

 tologic changes take place in the secreting structures of the gland. 

 At the close of a period of active secretion each acinus presents the 

 following features : the epithelial cells are short, cubic, nucleated, 

 and border a relatively wide lumen in which is to be found a 

 variable quantity of non-discharged milk. After the gland has rested 

 for some time, active metabolism again begins. The epithelial 

 cells grow and elongate ; the nucleus divides into two or three new 

 nuclei, and at the same time the cell becomes constricted; the inner- 

 portion is detached and is discharged into the lumen. Coincidentally 

 with these changes oil-globules make their appearance in the cell 

 protoplasm, some of which are discharged separately into the lumen, 

 while others remain for a time associated with the detached cell. 

 From these histologic changes it would appear that the caseinogen 

 and the fat-globules are metabolic products of the cell protoplasm, 

 and not derived directly from the blood. That lactose has a similar 

 origin appears certain from the fact that it is formed independently 

 of carbohydrate food. The water and inorganic salts are doubtless 

 secreted by a mechanism similar to that of all other secreting 

 glands. 



VASCULAR OR DUCTLESS GLANDS. 

 INTERNAL SECRETIONS. 



The metabolism of the body generally, as well as that of individual 

 organs, has been shown to be related not only to the physiologic 

 activity of such organs as the liver and pancreas, but also to the 

 activity of the so-called vascular or ductless glands. The influence 

 of the pancreas in regulating the production of glycogen by the liver, 

 and the influence of the liver in the maintenance of the general 

 metabolism through the production of glycogen and the formation 

 or urea, are now established facts. That the vascular or ductless 

 glands to an equal extent, though perhaps in a different way, as- 

 sist in the maintenance of physiologic processes, appears certain 

 from the results of animal experimentation. The explanation given, 

 and generally accepted at the present time, for the influence of these 

 glands is that they produce specific substances, which are poured into 



