610 ORGANS OF THE SENSES. 



Simple papillse, 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 papillse. The papillas, in structure, apparently resemble 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 

 papillae of the skin, for, according to Todd and Bowman, the surface of each is 

 studded with minute conical processes of the mucous membrane, which form 

 secondary papillee. In the papillse circumvallatse, the nerves are numerous and of 

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

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

 filiformes, their mode of termination is uncertain. 



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

 numerous follicles and glands. 



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

 numerous between the papillae circumvallatas 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 circum- 

 vallate papillae projecting into the muscular substance. Their ducts open either 

 upon the surface, or into the depressions round the large papilla?. 



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, but it is much thinner than in the skin, the inter- 

 vals between the large papilla) not being filled up by it, but each papilla has a 

 separate investment from root to summit. The deepest cells may sometimes be 

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

 subrnucous 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 

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

 making a vertical section through 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 aud intrinsic muscles. The former 

 have been already described ; they are the Hyo-glossus, Genio-hyo-glossus, Stylo- 

 glossus, Palato-glossus, and part of the Superior constrictor. The intrinsic mus- 

 cles 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 the hyoid 

 bone, the individual fibres being attached in their course to the submucous and 

 glandular structures. 



The inferior longitudinal fibres are formed by the Lingualis muscle, already 

 described (p. 261). 



