114 The Unity of the Organism 



The Conception of "Internal Secretions" 



The nature of the phenomena now to be considered, and 

 their significance for our discussion make it desirable to 

 think about these secretions from the broad standpoint first 

 stated, according to Bayliss, by Brown-Sequard and d'Arson- 

 val, namely, as materials produced by any living cells or 

 tissues which are discharged into the blood or lymph and 

 have specific effects on other parts or functions of the or- 

 ganism. Regarded thus it is now known that many cells of 

 the organism produce internal secretions. Although we are 

 more concerned with function than with structure in this 

 discussion, our purpose will be best served by beginning 

 with a morphological classification of the secretion-pro- 

 ducing cells. They may be divided into two categories, 

 those which are disposed into definite organs, or glands, the 

 ductless glands of long standing in anatomy; and those 

 which are not assembled in such organs. Knowledge of this 

 second class of cells is of recent date, and is fuller from the 

 functional than from the structural standpoint. The chief 

 glands, to which the name Endocrine has lately been given 

 by Schafer, are now so well known as hardly to need men- 

 tion. They are the thyroid apparatus, including the thy- 

 roids and the parathyroids, the suprarenal body, the pitui- 

 tary body, and probably the thymus and pineal bodies. Cells 

 now known to produce internal secretions but which are not 

 arranged in glands are certain cells of the pancreas scat- 

 tered among the pancreatic cells proper; certain cells of 

 the alimentary mucous membrane ; the interstitial cells of 

 the ovary and of the testis, and probably certain cells of 

 the placenta, of the mammary gland, and of the uterus. 



Following our usual course of making the treatment 

 merely illustrative rather than aiming at exhaustiveness, 

 our selection will include one example from each of these 

 groups. From the glandular category we take the thyroid 



