162 THE STRUCTURE OF MAN 



been developed out of the posterior part of the degenerating 

 sublingua. 



The study of Ontogeny has up to the present thrown no 

 light on the sublingua. 



Before quitting the tongue the papillee foliatse should be 

 mentioned. These, in Mammals, take the form of localised 

 systems of lamellae, situated on the postero-lateral tongue border, 

 and having their epithelium thrown into a series of flask-shaped 

 depressions. In Man these papillae vary much in form and size, 

 and since they are occasionally represented by but mere traces 

 they are evidently undergoing reduction. 



THYROID AND THYMUS 



These two organs are developmentally related to the pharyn- 

 geal region. 



The thyroid gland, in all Mammals in which it has been 

 examined, arises from two ventral outgrowths, one of which is 

 paired and the other unpaired. 



The unpaired constituent is closely connected ontogenetically 

 with the tongue which, during development, bridges over the 

 floor of the buccal cavity, enclosing a space, the wall of which 

 becomes changed into an epithelial vesicle. This is the unpaired 

 or median thyroid gland, and it for a time remains in com- 

 munication by means of its duct (the ductus thyroglossus) with 

 the posterior surface of the tongue, at its base of attachment. 

 When this duct closes, its orifice may become converted into the 

 so-called foramen ccecum of the adult, and therefore belongs to 

 the class of vestigial structures. The duct itself, as His has 

 shown, may often be retained in the adult for a length of 2^ or 

 more centimetres. Its existence explains the fact that the so-i 

 called middle lobe of the thyroid gland is occasionally prolonged! 

 upwards into a process, which often becomes constricted so asl 

 to form a series of from two to four longitudinally recurrentj 

 vesicles (bursse supra hyoidea and prsehyoidea). 



The paired portions, or the lateral lobes, of the thyroid gland 

 arise at the region of extreme posterior differentiation of the 

 visceral skeleton, by constriction of the primary floor of the 

 pharynx, near the laryngeal orifice. We have thus, here again, a 

 structure of epithelial origin. At a later stage the lateral and 

 median portions of the thyroid gland become approximated. 



