676 ORGANS OF SENSE. 



thickness and density of their epithelium ; they are covered with numerous 

 secondary papillae, are firmer and more elastic than the papilla of mucous mem- 

 brane generally, and often inclose minute hairs. 



Simple papillae, similar to those of the skin, are dispersed very unequally 

 among the compound forms, and exist sparingly on the surface of the tongue 

 behind the circumvallate variety, buried under a layer of epithelium. 



Structure of the papillae. The papillae apparently resemble in structure those 

 of the cutis, consisting of a cone-shaped projection of homogeneous tissue, 

 covered with a thick layer of squamous epithelium, and contain one or more 

 capillary loops, amongst which nerves are distributed in great abundance. If 

 the epithelium is removed, it will be found that they are not simple processes 

 like the papillaa of the skin, for the surface of each is studded with minute 

 conical processes of the mucous membrane, which form secondary papillae (Todd 

 and Bowman). In the papillaa circumvallatas, the nerves are numerous and of 

 large size ; in the papillaa fungiformes they are also numerous, and terminate in 

 a plexiform network, from which brush-like branches proceed ; in the papillae 

 filiformes, their mode of termination is uncertain. 



Besides the papillae, the mucous membrane of the tongue is provided with 

 numerous foll'des and glands. 



The follicles are found scattered over its entire surface, but are especially 

 numerous between the papillae circumvallatae and the epiglottis. 



The mucous glands (lingual), similar in structure to the labial and buccal, are 

 found chiefly beneath the mucous membrane of the posterior third of the 

 dorsum of the tongue. There is a small group of these glands beneath the tip 

 of the tongue, a few along the borders of the organ, and some in front of the 

 circumvallate papillae projecting into the muscular substance. Their ducts open 

 either upon the surface, or into the depressions round the large papillae. 



The epithelium is of the scaly variety like that of the epidermis. It covers the 

 free surface of the tongue, as may be easily demonstrated by maceration, or 

 boiling, when it can be detached entire. It is much thinner than in the skin ; 

 the intervals between the large papillae are not filled up by it, but each papillae 

 has a separate investment from root to summit. The deepest cells may some- 

 times be detached as a separate layer, corresponding to the rete mucosum, but 

 they never contain coloring matter. 



The tongue consists of two symmetrical halves, separated from each other, 

 in the middle line, by a fibrous septum. Each half is composed of muscular fibres 

 arranged in various directions, containing much interposed fat, and supplied by 

 vessels and nerves; the entire organ is invested by mucous membrane, and a 

 submucous fibrous stratum. The latter membrane invests the greater part of 

 the surface of the tongue, and into it the muscular fibres are inserted that pass 

 to the surface. It is thicker behind than in front, and is continuous with the 

 sheaths of the muscles attached to it. 



The fibrous septum consists of a vertical layer of fibrous tissue, extending 

 throughout the entire length of the middle line of the tongue, from the base to 

 the apex. It is thicker behind than in front, and occasionally contains a small 

 fibro-cartilage, about a quarter of an inch in length. It is well displayed by 

 making a vertical section across the organ. Another strong fibrous lamina, 

 termed the hyo-glossal membrane, connects the under surface of the base of the 

 tongue to the body of the hyoid bone. This membrane receives, in front, some 

 of the fibres of the Genio-hyo-glossi. 



Each half of the tongue consists of extrinsic and intrinsic muscles. The 

 former have been already described; they are theHyo-glossus, Genio-hyo-glossus, 

 Stylo-glossus, Palato-glossus, and part of the Superior Constrictor. The intrinsic 

 muscles are the Superior Longitudinal, Inferior Longitudinal, and Transverse. 



The superior longitudinal fibres (lingualis superficialis) form a superficial 

 stratum of oblique and longitudinal fibres on the upper surface of the organ, 

 beneath the mucous membrane, and extend from the apex backwards to th.3 



