THE ORGANS OF SPECIAL SENSE. 



THE Organs of the Senses are five in number, viz., those of Touch, of Taste, 

 of Smell, of Hearing, and of Sight. The skin, which is the principal seat 

 of the sense of touch, will be described in the section on General Anatomy. The 

 remaining four are the Organs of Special Sense. 



THE TONGUE. 



The Tongue is the organ of the special sense of taste. It is situated in the 

 floor of the mouth, in the interval between the two lateral portions of the body of 

 the lower jaw. 



Its base or root is directed backward, and connected with the os hyoides by 

 the Hyo-glossi and Genio-hyo-glossi muscles and the hyo-glossal membrane ; with 

 the epiglottis by three folds (glosso-epiglottic] of mucous membrane ; with the soft 

 palate by means of the anterior pillars of the fauces ; and with the pharynx by 

 the Superior constrictors and the mucous membrane. Its apex or tip, thin and 

 narrow, is directed forward against the inner surface of the lower incisor teeth. 

 The under surface of the tongue is connected with the lower jaw by the Genio-hyo- 

 glossi muscles ; from its sides the mucous membrane is reflected to the inner sur- 

 face of the gums ; and from its under surface on to the floor of the mouth, where, 

 in the middle line, it is elevated into a distinct vertical fold, \hefrcenum linguae. 

 To the outer side of the fraenum is a slight fold of the mucous membrane, the 

 plica fimbriata, the free edge of which exhibits a series of fringe-like processes. 



The tip of the tongue, part of the under surface, its sides, and dorsum are free. 



The dorsum of the tongue is convex, marked along the middle line by a raphe", 

 which divides it into symmetrical halves ; this raphe" terminates behind, about an 

 inch from the base of the organ, in a depression, the foramen caecum, from which 

 a shallow groove, the sulcus terminates of His, runs outward and forward on each 

 side to the lateral margin of the tongue. The part of the dorsum of the tongue in 

 front of this groove, forming about two-thirds of its upper surface, is rough and 

 covered with papillae ; the posterior third is smoother, and contains numerous 

 muciparous glands and lymphoid follicles. 



Structure of the Tongue. The tongue is partly invested by mucous membrane 

 and a submucous fibrous layer. It consists of 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 (page 325), containing much inter- 

 posed fat, and supplied by vessels and nerves. 



The mucous membrane invests the entire extent of the free surface of the 

 tongue. On the dorsum it is thicker behind than in front, and is continuous with 

 the sheath of the muscles attached to it, through the submucous fibrous layer. On 

 the under surface of the organ, where it is thin and smooth, it can be traced on 

 each side of the fraenum through the ducts of the submaxillary and the sublingual 

 glands. As it passes over the borders of the organ it gradually assumes its papil- 

 lary character. 



The structure of the mucous membrane of the tongue differs in different parts. 

 That covering the under surface of the organ is thin, smooth, and identical in 



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