1867.] 



Mr. Callender on the Thyroid Body. 



185 



second on the right side, measuring one-tenth of an inch from end to 

 end, and a third on the left side, measuring one-fourteenth of an inch. 



Relations luith the Thymus. 



That the connexions of the thyroid are from the first with the larynx 

 and trachea, rather than with the thymus, is rendered more probable by 

 the appearances observed in the young of animals. In a foetal rabbit 

 (fig. 3)*, eight-tenths of an inch long, the thymus may be seen to con- 

 sist of two lobulated masses, lying side by side just above the heart 

 and its great vessels, broader at the base towards the thorax, diverging a 

 little as they pass upwards, and ending in the root of the neck by a 

 somewhat pointed extremity. The trachea in the middle line is sur- 

 mounted by the larynx, but at its upper extremity is a minute elevation, 

 contrasting by its pale colour with adjacent parts ; and connected with this 

 are two divergent ridges, of the same pale colour, which embrace, horse- 

 shoe fashion, the lower portion of the larynx, tapering as they ascend, 

 and resembling, so far as the mere look is concerned, the division of 

 the trachea at its lower extremity into the two bronchi. In a foetal pig 

 (fig. 4)t, one inch and two-tenths in length, the thyroid is notched 

 below, thus acquiring, though somewhat indistinctly, a three-lobed ap- 

 pearance ; and here also, whilst firmly attached to the trachea, it is no way 

 visibly in relation with the thymus. 



One cannot but be attracted by this connexion with the trachea, on 

 which tube the thyroid (even if it be not developed from the membranous 

 air- tube) buds and attains some little size, a formation reminding one of that 

 of the lungs coming out from the front wall of the oesophagus, that is, from 

 the trachea, and of the view^ of Mr. Simon, as afterwards expressed by the 

 editors of Cuvier J respecting the thyroid, **C'est la fausse branchie, bran- 

 chiole des poissons." Indeed, from its relation to the air-tube during the 

 early period of life, or in fish to the vertebral or hyoidal extremity of the 

 gill, from its curious alternation with the supplementary gill of i5roussonet§, 

 and from its structure (Kolliker), it may be not inaptly referred to as a 

 pseudo-lung rather than as an associate with the thymus and the so-called 

 ductless glands. 



I may add that, in the human foetus, no distinct evidence of the thyroid 

 appears to me to exist before the sixth week, up to which time it cannot, I 

 believe, be isolated from the structures in the front of the neck ; it seems 

 to come out from the blastema in the form of a mass in front of the 

 trachea, which quickly acquires an imperfectly lobed condition, but I have 

 not been able to distinguish at any period, during development of size, 

 three completely distinct parts. 



* See also specimen No. 8. 



t See also specimen No. 9. 



X Le9ons d'anatomie compar6e, 2meedit. torn viii. 1846, p. 678. 

 § Simon, Philosophical Transactions, 1844, p. 801. 



