

THE MOUTH 1125 



There are, however, considerable variations in these times; thus, according 

 to Holt: 



At the age of 1 year a child should have 6 teeth. 

 " 1| years " " 12 " 



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Calcification of the permanent teeth proceeds in the following order in the 

 lower jaw (in the upper jaw it takes place a little later): the first molar, soon 

 after birth; the central and lateral incisors, and the canine, about six months 

 after birth; the premolars, at the second year, or a little later; the second molar, 

 about the end of the second year; the third molar, about the twelfth year. 



The eruption of the permanent teeth takes place at the following periods, the 

 teeth of the lower jaw preceding those of the upper by short intervals : 



First molars 6th year. 



Two central incisors 7th year. 



Two lateral incisors 8th year. 



First premolars ... 9th year. 



Second premolars 10th year. 



Canines llth to 12th year*. 



Second molars 12th to 13th year. 



Third molars 17th to 25th year. 



Toward the sixth year, before the shedding of the deciduous teeth begins, there 

 are twenty-four teeth in each jaw, viz., the ten deciduous teeth and the crowns 

 of all the permanent teeth except the third molars. 



The Tongue (Ungua). The tongue is the principal organ of the sense of taste, 

 and an important organ of speech ; it also assists in the mastication and deglutition 

 of the food. It is situated in the floor of the mouth, within the curve of the 

 body of the mandible. 



Its Eoot (radix lingua; base) (Fig. 954) is directed backward, and connected 

 with the hyoid bone by the Hyoglossi and Genioglossi muscles and the hyoglossal 

 membrane; with the epiglottis by three folds (glossoepiglottic) of mucous membrane; 

 with the soft palate by the glossopalatine arches; and with the pharynx by the 

 Constrictores pharyngis superiores and the mucous membrane. 



Its Apex (apex linguae; tip), thin and narrow, is directed forward against the 

 lingual surfaces of the lower incisor teeth. 



Its Inferior Surface (fades inferior lingua; under surface) (Fig. 1013) is connected 

 with the mandible by the Genioglossi; the mucous membrane is reflected from it 

 to the lingual surface of the gum and on to the floor of the mouth, where, in the 

 middle line, it is elevated into a distinct vertical fold, the frenulum linguae. On 

 either side lateral to the frenulum is a slight fold of the mucous membrane, the 

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

 processes. 



The apex of the tongue, part of the inferior surface, the sides, and dorsum are 

 free. 



The Dorsum of the Tongue (dorsum lingua) (Fig. 1014) is convex and marked by 

 a median sulcus, which divides it into symmetrical halves; this sulcus ends behind, 

 about 2.5 cm. from the root of the organ, in a depression, the foramen cecum, 

 from which a shallow groove, the sulcus terminalis, runs lateralward and forward 

 on either side to the margin of the tongue. The part of the dorsum of the tongue 

 in front of this groove, forming about two-thirds of its surface, looks upward, and 

 is rough and covered with papillae; the posterior third looks backward, and is 



