CHAPTER XVI 



THE DUCTLESS GLANDS ENDOCKIN GLANDS 

 (Organs of Internal Secretion] 



Under this heading it will be convenient to consider the suprarenal, 

 thyroid,, parathyroid, thymus, carotid, and coccygeal glands, the hypoph- 

 ysis cerebri, the epiphysis cerebri and the paraganglia. This group 

 of organs properly includes also the pancreatic islets, the interstitial 

 cells of the testis and the ovary, and the corpora lutea already described. 



I. THE SUPRARENAL GLANDS 



THE SUPRARENAL GLANDS (bodies or capsules; also called adrenals] 

 are two flattened irregular glandular masses situated close to the cranial 

 extremity of each kidney, but without organic or genetic relationship 

 with the renal system. On section the adrenal is seen to be readily 

 divisible into a bright yellow or brownish-yellow cortex and a more 

 vascular, and hence darker and somewhat reddish, medulla, whose central 

 portion transmits several large veins which make their exit from an 

 indentation in the anterior surface of the organ, known as the hiluin. 



Development and Function. The suprarenal gland is a composite 

 structure formed by the intimate association of two embryologieally dis- 

 tinct anlages: one, the cortical component, of mesodennal, the other or 

 medullary component, of ectodermal origin. The cortical component arises 

 as a series of buds from the celomic epithelium covering the medial upper 

 surface of the cephalic third of the Wolffian body; the medulla develops 

 from cells which have migrated from the abdominal sympathetic plexus 

 (celiac plexus), and which elaborate peculiar granules having an affinity 

 for chromium, hence called chromaffin (or plieochrome] cells. The onto- 

 genetic process recapitulates almost precisely the phylogenetic history of 

 the suprarenals : In fishes the homologues of the two mammalian com- 

 ponents remain separate, and the cortical representative includes a series 

 of bodies, the interrenal ~bodies or organs; the medullary representative, 



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