1126 THE DIGESTIVE SYSTEM. 



directed backwards, and corresponds to a depression on the surface of the 

 tongue, the foramen caecum, whilst its diverging limbs pass laterally and forwards 

 towards the attachments of the glosso-palatine arch. The foramen caecum is 

 the remains of a tubular downgrowth formed early in embryonic life, in the 

 floor of the primitive pharynx, from which the isthmus of the thyreoid glaiid 

 is developed (see p. 44). 



The anterior portion presents a velvety surface and is covered with innumerable 

 papillae; the taste-buds are situated in it, and it is horizontal in position. It 

 is developed from the tissues of the floor of the pharynx behind the first visceral 

 arch. The posterior portion, on the other hand, has a smooth glistening surface, 

 contains numerous serous glands and small lymph follicles, and is more vertical 

 in position. It is developed from the tissue covering the ventral ends of the 

 second and third visceral arches (see p. 45). 



The anterior or oral portion of the dorsum linguae (Fig. 890) is convex, 

 both from before backwards and from side to side in the resting condition of 

 the organ (Fig. 893). It usually presents a slight median depression, sulcus 

 medianus, in the form of an irregular crease, which ends posteriorly near the 

 foramen caecum. The mucous membrane of this portion of the dorsum is thickly 

 covered with the prominent and numerous papillae linguales which give this portion 

 of the tongue its characteristic appearance. 



On the pharyngeal part of the tongue there are also small papillary projections of the 

 corium, but the epithelium fills up all the intervals between the papillae, and, as it were, levels 

 off the surface, so that none are visible to the eye as projections above the general level. Over 

 the anterior part of the tongue, on the contrary, the projections of the corium are large and 

 prominent, and the intervals between them, while they are covered, yet are not filled up, by the 

 epithelium, so that the projections stand out distinctly and independently, and in places attain a 

 height of nearly 2 mm. above the general surface. 



The posterior or pharyngeal portion of the dorsum linguae (Fig. 889), nearly 

 vertical in direction, forms the greater portion of the anterior wall of the oral 

 part of the pharynx (Fig. 890). Its surface is free from evident papillae, but is 

 thickly studded with rounded projections, 'each presenting, as a rule, a little 

 pit, visible to the naked eye, at its centre ; the great majority of these folliculi 

 linguales (lingual follicles, Fig. 889), are similar to the lymph follicles found in 

 the palatine tonsils ; some few are said to be mucous glands ; all are covered by a 

 smooth mucous membrane, and they combine to give to this region a characteristic 

 nodular appearance. To this collection of follicles the name tonsilla lingualis 

 is applied. 



The mucous membrane of this portion of the tongue is separated from the 

 muscular substance by a submucous layer in which the lymph follicles and the 

 mucous glands lie embedded (Fig. 893). At the sides it is continuous with 

 the tunica mucosa covering the palatine tonsils and the side wall of the pharynx ; 

 whilst posteriorly it is reflected on to the front of the epiglottis, forming in the 

 middle line a prominent fold, the plica glosso-epiglottica (Fig. 889), at each side 

 of which is a wide depression, the vallecula. 



On each side is a pharyngo-epiglottic fold, which passes from the side of the epiglottis, 

 upwards along the wall of the pharynx, upon which it is soon lost. 



Papillae of the Tongue (Fig. 891). These are formed by variously shaped 

 projections of the corium of the mucous membrane, covered by thick caps of epi- 

 thelium. They are of three main varieties : 1, Conical and filiform (jpapillce conicce, 

 p. filiformes) ; 2, Fungiform and lenticular (papillce fungiformeset p. lenticulares}] 

 and 3, Vallate and foliate (papillce vallatce et p. foliatce). 



The conical and filiform papillae (Fig. 891) are the smallest and most numerous, 

 forming as they do a dense crop of minute projections all over the anterior two- 

 thirds of the dorsum, and also upon the superior part of the margin and tip, of the 

 tongue. Posteriorly they are arranged in divergent rows running laterally and 

 forwards from the raphe, parallel to the limbs of the sulcus terminalis. More 

 anteriorly, the rows become nearly transverse, and near the tip irregular. Each 



