THE TONGUE AND THE THYROID BODY. 557 



marked in the adult : it is always indicated, in the median plane, 

 by the foramen caecum (Fig. 242, fk). The line of circum- 

 vallate papillae, which appears during the third month, lies 

 immediately in front of this groove, and therefore in the part 

 of the tongue formed from the tuberculum impar : immediately 

 in front of the foramen caecum, and sometimes surrounding it, is 

 a single, very deeply depressed circumvallate papilla. 



The double origin of the tongue is indicated by its nerve 

 supply; the body and tip of the tongue, developed from the 

 tuberculum impar, are supplied by the gustatory branch of the 

 trigeminal nerve ; while the root and sides of the tongue, 

 developed from the transverse ridge, are supplied by the glosso- 

 pharyngeal. It must be noted, however, that in order to reach 

 the circumvallate papillae the branches of the glosso-pharyngeal 

 nerve have to overstep the boundary between the two parts of 

 the tongue, and invade the part formed from the tuberculum 

 impar. 



5. The Thyroid Body. 



The thyroid body is formed from three independently arising 

 rudiments, which remain distinct until a rather late stage in deve- 

 lopment : (i) a middle thyroid rudiment (Fig. 239, th), which is a 

 deep pit commencing at the foramen caecum, at the junction of 

 the body and root of the tongue, and extending downwards and 

 backwards in the floor of the mouth ; and (ii) a pair of lateral 

 thyroid rudiments, which are outgrowths of epithelium from the 

 floor of the mouth at the sides of the larynx, in close relation 

 with the third branchial pouches. 



The middle thyroid rudiment, which appears about the middle 

 of the fourth week, consists at first of a short tubular duct, 

 which divides at its blind end into right and left lobes (Fig. 239, 

 th). During the fifth week the median duct, or thyro-glossal 

 duct, elongates rapidly, growing downwards and backwards until 

 its bifurcated distal end lies opposite the larynx, or upper end 

 of the trachea. During this rapid growth the duct usually loses 

 its lumen, and becomes a solid rod of epithelial cells extending, 

 in the median plane, from the foramen cscuin to the trachea. 



Towards the end of the fifth week, this epithelial cord 

 usually becomes broken up in the middle part of its course into 

 a number of detached fragments ; and a little later it becomes 



