THE ENDOCRINAL ORGANS 455 



In the amphibia 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 amphibia the chromafiEin tissue surrounds the interrenal 

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

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

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

 two tissues are mixed as in amphibia. Strands and clusters of chromaffin 

 cells are embedded within the interrenal tissue. 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 medul- 

 lary (chromaffin) tissue. 



THE THYROID GLAND 



Anatomy. The thyroid gland of man is a bilobed brownish organ 

 closely apposed to the trachea just below the larynx. The lateral lobes 

 are generally connected by an isthmus across the trachea, so that the 

 shape of the gland varies from a "U" to an "H," depending upon the 

 relative position of the isthmus. Not infrequently, especially in younger 

 persons, there is a median pyramidal lobe. The accessory thyroids which 

 sometimes occur are usually remnants of this pyramidal lobe. There is, 

 also, occasionally another accessory gland beneath the upper end of the 

 sternum. 



Although the size of the thyroid varies greatly in different individuals, 

 the average weight is thirty-four grams. The connective tissue capsule 

 surrounding the gland is a double membrane. Suspensory ligaments of 

 connective tissue attach the thyroid to the tracheal and laryngeal 

 cartilages. 



The blood supply of the thyroid, like that of the adrenals, is exception- 

 ally abundant. Four, and frequently five, arteries connect with the 

 organ; seven veins drain the blood away. The arteries are the paired 

 superior and inferior thyroid, and the ocasional thyroidea ima. On the 

 surface of the gland, a plexus of veins gives rise to paired, middle, and 

 inferior thyroid veins and to the unpaired thyroidea ima. Lymphatics 

 are abundant. The nerve supply is sympathetic. 



Histology. The thyroid is formed of numerous spherical masses of 

 glandular tissue, separated from one another by connective tissue parti- 

 tions. The glandular tissue consists of rounded follicles, 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-like colloid material which has a strong affinity 



