168 



THYKOID GLAND 



The superior cornu is at first continuous with the great horn of the hyoid on each 

 side, and the cartilago triticea in the lateral thyro-hyoid ligament of the adult is a 

 remnant of the cartilaginous connexion. 



The thyroid gland is developed partly from a median diverticulum of the 

 pharyngeal entoderm opposite the ventral ends of the second visceral pouches 

 (fig. 203, A, Sd), partly from lateral diverticula of the posterior walls of the 

 fourth visceral pouches. The median diverticulum in most animals early becomes 

 separated from the pharyngeal entoderm, and is thus converted into an island of 

 epithelium imbedded in mesenchyme. In the human embryo (fig. 211, A, thr) 

 it remains for some time in the form of a hollow bifid vesicle, which is connected 

 with the upper surface of the tongue] by a small duct (ductus thyreoglossus, d) ; 

 subsequently, however, the vesicle becomes solid, and the duct is obliterated and 

 disappears, with the exception of a small portion near the orifice, which becomes 

 converted into the foramen ccecum of Morgagni, f.c. 



Occasionally even in the adult a comparatively long duct is found, leading 

 downwards and backwards from the foramen caecum. This, which has been 

 termed the ductus lingualis, is the remains of the original thyrolingual duct 



fc. 



FIG. 211. SKETCHES SHOWING THE CONDITION or THE THYROID AND THYMUS GLANDS IN A 



HUMAN EMBRYO OP ABOUT FIVE WEEKS. (His.) 



A, profile sketch from the left side. B, frontal sketch from behind. 



t, tongue: fc, foramen caecum; d, ductus thyreoglossus; ep, epiglottis; opposite /, larynx; 

 tr, trachea; oe, oesophagus; thr, median rudiment of thyroid; thr 1 , lateral rudiment of thyroid; 

 thm, developing thymus, seen on the left side of B to be connected with a visceral cleft ; ao (in B), 

 ascending aorta ; ao', descending aorta ; c, carotid. 



connecting the median part of the thyroid Vith the tongue. It may further happen 

 that the lower part of this connexion also remains in the shape of a tubular 

 prolongation of the median portion of the thyroid towards the root of the tongue 

 (ductus thryoideus ; when well developed this portion forms the pyramid). The 

 so-called accessory thyroid bodies (suprahyoid, prsehyoid glands, &c.) which are 

 occasionally found near the hyoid bone are also referable to the thyrolingual 

 duct (His). The mesial rudiment soon becomes enlarged into a lobed mass of 

 considerable size, which, as the neck elongates, assumes the shape of a horse- shoe 

 passing across the front of the trachea (figs. 212-214). It becomes converted into 

 ramifying and anastomosing cell- cylinders, between which vascular connective tissue 

 is developed. The cell-cylinders subsequently become hollowed out, and finally 

 are subdivided by growth of the connective tissue into small vesicles, which 

 gradually become larger from accumulation of colloid in their interior. 



The two lateral diverticula, which assist in the formation of the thyroid body, 

 spring from the fourth visceral pouches (fig. 213). They have at first the 

 appearance of simple thick- walled saccular glands (fig. 216) partially encircling 

 the developing larynx. In front of this they come into connexion with the 

 median rudiment, and eventually blend with it. Like that rudiment, they become 



