THE TONGUE 1219 



The Eruption of the Permanent Teeth takes place at the following periods, the teeth of the 

 lower jaw preceding those of the upper by a short interval : 



6J years, first molars. 10th year, second bicuspid. 



7th year, two middle incisors. llth to 12th year, canine. 



8th year, two lateral incisors. 12th to 13th year, second molars. 



9th year, first bicuspid. 17th to 21st year, third molars. 



THE TONGUE (LINGUA) (Fig. 938). 



The tongue is a very mobile muscular organ, undergoing changes in length and 

 width at every contraction of its muscle. It is one of the organs of the special 

 sense of taste, and is also an organ of speech, and assists in insalivation, masti- 

 cation, and deglutition. It is situated in the floor of the mouth, in the interval 

 between the two lateral portions of the body of the mandible, and when at rest 

 is about three and one-half inches (8.75 cm.) in length. We describe the body, 

 base, apex, dorsum, margin, and inferior surface. 



The body (corpus linguae) forms the great bulk of the organ and is composed 

 of striated muscle. 



The base or root (radix linguae) is directed backward and connected with the 

 hyoid bone by the Hyoglossi and Geniohyoglossi muscles and the hyoglossal mem- 

 brane; with the epiglottis by three folds, glossoepiglottic folds, of mucous membrane; 

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

 pharynx by the Superior constrictor muscles and the mucous membrane. 



The apex or tip (apex linguae) is free, thin, and narrow, and is directed forward 

 against the inner surface of the lower incisor teeth. 



The dorsum of the tongue (dorsum linguae) of a living person, when at 

 rest, is markedly arched from before backward. On the dorsum is a median 

 longitudinal raphe (sulcus medianus linguae). This slight depression terminates 

 posteriorly in the depression known as the foramen cecum (foramen caecum linguae 

 [Morgagnii]), from which a shallow-shaped groove, the sulcus terminalis 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, known as the anterior, 

 apical, or oral part, forming about two-thirds of its upper surface, looks upward, 

 is practically horizontal, and is rough and covered with papillae; the posterior or 

 basal third of the dorsum is vertical and looks backward, is smoother, and contains 

 numerous muciparous glands and lymphoid follicles. 



The margin of the tongue (margo lateralis linguae) is free in front of the anterior 

 arch of the palate. Just in front of the arch are several vertical folds, the folia 

 linguae. 



The under or inferior surface (fades inferior linguae) of the tongue is connected 

 with the mandible by the Geniohyoglossi muscles; from its sides the mucous mem- 

 brane is reflected to the inner surface 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, the frenum linguae (frenulum linguae). To each side of the frenum 

 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. 



Structure of the Tongue. The tongue is partly invested by mucous membrane and a sub- 

 mucous fibrous layer surrounding the central muscle tissue. It consists of symmetrical halves, 

 separated from each other, in the middle line, by a fibrous septum. Each half is composed of 

 muscle fibres arranged in various directions (p. 394), containing many glands and much inter- 

 posed fat, and supplied by vessels and nerves. 



The mucous membrane (tunica mucosa linguae) 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 



