142 HUMAN PHYSIOLOGY. 



rest. Coincidentally with these periods, certain histologic 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 asso- 

 ciated with the detached cell. From these histologic changes it would ap- 

 pear 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 7 called 

 vascular or ductless glands. The influence of the pancreas in regulating the 

 oxidation of sugar, 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, assist in the main- 

 tenance 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 the blood or lymph and carried direct to 

 the tissues, to the activities of which they appear to be essential; for without 

 these substances the nutrition of the tissues declines and in a short time a 

 fatal termination ensues. 



Inasmuch as these partly unknown substances are formed by cell activity 

 and are poured into the interstices of the tissues, they have been termed 

 "internal secretions." Though the term internal secretions is applicable 

 to all substances which arise in consequence of tissue metabolism, and which 



