1112 



DIGESTIVE SYSTEM 



uted to the root of the tongue, includmg also the vallate and foliate papillae (nerve of taste); 

 and the superior laryngeal branch of the vagus, which supplies a small area near the epiglottis. 



Development. — The development of the tongue is quite complicated. In general, the body 

 of the tongue is derived from the region corresponding to the ventral portion of the first arch, 

 just behind the mandible. It does not develop from the tuberculum impar, however, which is 

 a transitory structure (Hammar). The epithelium of the body of the tongue is probably of 

 ectodermal origin. The root of the tongue develops from the corresponding lower portion of the 

 second or hyoid arch, and its epithelium is endodermal in origin. The transverse groove be- 

 tween the two arches later becomes the sulcus terminalis. At the middle of this groove there 

 is an ingrowth of the epithelium to form the anlage of the thyreoid gland. The foramen ccecum 

 and the occasional ductus linguaHs represent persistent portions of the thyreoid duct. The third 

 arch does not appear to enter into the formation of the tongue, but forms the epiglottis 

 (Hammar). 



The musculature of the tongue appears to develop from the mesenchyme in situ although 

 its innervation from the hypoglossal would indicate a derivation from the occipital myotomes. 

 A pair of premuscle masses appears in the 9 mm. embryo, the individual extrinsic muscles being 

 distinguishable at 14 mm., and the intrinsic at 20 mm. (W. H. Lewis). The glands appear 

 in the fourth foetal month as solid epithelial downgrowths which later acquire a lumen. The 

 mucous glands appear first, the serous slightly later. Longitudinal folds in the mucosa of the 



Fig. 863.— The Salivary Glands. 



Accessory parotid 



Duct of accessory 

 parotid 



Duct of parotid 



Bristle inserted 

 into duct 



Frenulum linguae 



Major sublingual 

 duct 



Sublingual gland"^ 



Duct of submaxil 

 lary gland 



Mylo-hyoid musclC'^ 



Anterior belly of 

 digastric muscle 



Parotid gland 



Masseter muscle 



Sterno-mastoid 

 muscle 



' Posterior belly of 

 digastric muscle 



Lingual nerve 



Submaxillary 

 gland, 

 drawn backward 



Loop of fascia 



Hyoid bone 



Deep portion of submaxillary gland 



radix appear in the third or fourth foetal month (Jurisch). The lymphoid tissue appears some- 

 what later as aggregations in the lamina propria, chiefly around the gland-ducts. From the 

 beginning, the lymphoid structures are subject to marked individual variations. Character- 

 istic, well-developed lingual follicles do not appear until .some time after birth, however (Jurisch). 

 Of the lingual /japillw, the fungiform appear at the end of the third foetal month, followed 

 shortly by the filiform and vallate. The formation of the papilhc is not completed at birth, 

 however, since they later undergo changes in number, form, size and arrangement. The 

 foliate papilla; appear al)out the fifth fcetal month. They are best developed in infants, under- 

 going retrogressive changes in the adult (Stahr). The same is true of the plic;e fimbriatie. 



Variations. — Of the; manifold variations in the structure of the tongue, some have already 

 been mentioned. Additional mucous glands sometimes occur along the margin of the tongue 

 (completing Oppcl's "glandular ring"). In "tongue-tied" individuals, the frenulum is ab- 

 normally short. A forked tongue (normal in some animals) is a rare congenital anomaly. 

 Another rare variation is the so-called "hairy" tongue, due to hypertrophy of the filiform 

 papilhe. While tlu; V-shaped arrangement of the vallate i)aj)ilhc is typical, the Y-form (two 

 to four j)apilla! in the me<lian line forming the stem of the Y) is nearly as frequent. Indeed, 

 in some of the coloured races the latter typc! seems to predominate. The sulcus terminalis 

 and foramen cjecum are oft(!n indistinct and sometimes absent. 



Comparative. — The tongue of fishes and lower amphibia contains neither glands nor in- 

 trinsic iiiusciilature. Among higher vertebrates, the tongue varies exceedingly in form and 

 structure, but always contains intrinsic musculature and mucous glands, 'liie latter primi- 

 tively form a ring around the margin and root of the tongue (Oppel). The serous glands occur 

 only in mammals, and are associated closely with the papilhe bearing taste-buds. 



