THE ENDOCRINAL ORGANS 329 



system. This substance, the chemical nature of which is unknown, has 

 been called sympathin. 



Corresponding with its double function and its division into cortex and 

 medulla, each adrenal has a double origin. The cortex develops from 

 mesoderm, while the medulla is derived from sympathetic ganglia and is 

 therefore ectodermal in origin. In embryos of the third month, the 

 adrenals are more conspicuous organs than the kidneys, but as develop- 

 ment proceeds, they fail to keep pace with the enlargement of the body. 



Adrenal glands are Umited to vertebrates. In cyclostomes, both the 

 cortical material and the chromaffin arepresent, but are spatially separated. 

 The cortical substance is represented by the interrenal bodies, which 

 lie throughout the length of the body-cavity near the postcardinal veins. 

 The chromaffin cells, on the other hand, are arranged as strands along the 

 dorsal aorta. In the elasmobranchs, the interrenal bodies tend to 

 aggregate in the posterior part of the body-cavity, while the chromaffin 

 cells are arranged in metameric groups near the sympathetic ganglia. 



In the amphibians the organization of the adrenals is intermediate 

 between that of elasmobranchs and that of amniotes. Chromaffin and 

 interrenal cells lie in close proximity to one another. The two kinds of 

 tissue are usually interspersed and extend along the surface of the meso- 

 nephroi. In some amphibians the chromaffin tissue surrounds the inter- 

 renal cells. The relations of the two tissues vary considerably in reptiles. 

 In some, though not in all, the chromaffin cells, as in fishes, are separate 

 from the interrenal bodies. In crocodiles and tortoises, however, the two 

 tissues are mixed as in amphibians. In birds also the two kinds of tissue 

 are intermingled. Finally, in mammals, the interrenal tissue forms a 

 cortex which encloses the chromaffin cells as a medulla. The quantity of 

 cortical tissue in mammals greatly exceeds that of medullary (chromaffin) 

 tissue. 



Th3rroid Gland. The thyroid gland of man is a bilobed brownish organ 

 closely apposed to the trachea just below the larynx. Although the size 

 of the thyroid varies greatly in different individuals, the average weight is 

 thirty-four grams. The blood supply of the thyroid, like that of the 

 adrenals, is exceptionally abundant. Four, and frequently five, arteries 

 connect with the organ; seven veins drain the blood away. The nerve 

 supply is sympathetic. 



The thyroid is formed of numerous spherical masses of glandular tissue, 

 separated from one another by connective-tissue partitions. The glandu- 

 lar tissue consists of rounded folUcles, each enclosed by a single layer of 

 cuboidal epithelium. Loose connective tissue filled with blood-vessels 

 and lymphatics binds the follicles together. Each follicle is filled with 

 jelly-Uke colloid material which has a strong affinity for acid dyes such as 

 eosin. 



