THYROID GLAND OF MAMMALIA. 563 



351. Thyroid of Mammalia.- The representative beginnings 

 of the vasoganglion commonly known as the e thyroid gland ' 

 are noticed in vol. i. p. 564 (Fishes, Reptiles), and in vol. ii. 

 p. 230 (Birds) : but this organ is recognisable, without am- 

 biguity, only in the present class. Here it is locally related to 

 the windpipe, and has received its name from its proximity to 

 the shield-shaped cartilage of the larynx in the human subject. 

 It consists of a pair of oblong, rounded masses ; in some, espe- 

 cially higher gyrencephalous Mammals, united as in Man by a 

 transverse band of like substance crossing the sternal aspect of 

 the air-tube. 



The proper tunic of the thyroid is a thin layer of condensed 

 areolar tissue, from the inner surface of which proceed septal or 

 trabecular processes, partitioning its substance into lobules, and 

 ultimately into minute bags of vesicles. The analogy to the struc- 

 ture of the spleen is close, but the frame-work is much less dense 



and fibrous : and the vesicular structure, instead of receivino; the 



^j 



blood directly, is filled with a solution of nbro-albiiminoid, pro- 

 tcine, or myeline substance derived therefrom. The quantity of 

 blood sent to the thyroid is much more than would be needed for 

 mere nutrition : it is derived from arteries, not constantly rising 

 just beyond the points where the arteries to the brain are given 

 off from the large trunks, but varying according as the length of 

 the neck in Mammals may affect the relative position of the 

 thyroid to those trunks : thus in the Giraffe and most Ungulates 

 the arteries supplying the thyroids come off from the contiguous 

 part of the carotids. There may be two or three branches from 

 the common carotid (Lutra) ; and the distinction between ( lower 

 thyroid arteries ' from the subclavian, and ( upper thyroid arteries ' 

 from the ectocarotid, hardly begins to be established before the 

 Quadrumanous order is reached. The ultimate ramifications of 

 these arteries form close-meshed plexuses upon the limitary mem- 

 brane or capsule of the vesicles ; such capillaries present a diameter 

 of from ^-pL-^ th to -j-^-o-th of an inch. The blood is returned by 

 veins joining in most Mammals the external jugular ; and in 

 Quadrumana and Man the internal jugular : but with varieties in 

 this respect. 



The effect of ' formifaction,' or assumption of shape and defin- 

 able size, by the ' colloid,' ' proteine.' or ( myeline ' elements of 

 the solution filling the thyroid vesicles, is shown in the sections of 

 such from the Hedgehog, fig. 435, and the Bullock, fig. 437, 

 and in portions of such lining, or adherent formed matter, from 

 the thyroid vesicles of a Rabbit, fig. 436. In these instances 



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