932 



THE ORGANS OF DIGESTION. 



periosteum of the alveolar processes and surrounding the necks of the teeth. They 

 are covered by smooth and vascular mucous membrane, which is remarkable for its 

 limited sensibility. Around the necks of the teeth this membrane presents numer- 

 ous fine papillae. The deep fibrous layer is continuous with the periosteum lining 

 the alveoli, (the pericementum). 



THE TEETH. 



The human subject is provided with two sets of teeth, which make their appear- 

 ance at different periods of life. The first set appear in childhood, and are called 



the temporary, deciduous, or milk teeth. The 

 second set are named permanent. 



The temporary teeth are twenty in num- 

 ber four incisors, two canine, and four 

 molars, in each jaw (Fig. 553). 



The permanent teeth are thirty-two in 

 number four incisors (two central and two 

 lateral), two canines, four bicuspids, and six 

 molars in each jaw (Fig. 554). 



General Characters. Each tooth consists 

 of three portions : the crown, or body, pro- 

 jecting above the gum; the root, or fang, 

 entirely concealed within the alveolus ; and 

 the neck, the constricted portion, between the 

 root and crown. 



The surfaces of a tooth are named thus : that which looks toward the lips is 

 the labial; that toward the tongue is the lingual ; that toward the mesial line, 

 proximal; that away from the same, distal; that toward the cheek, the buccal 

 surface. This applies to the roots as well as to the crowns of teeth. 



FIG. 553. Deciduous teeth. Left side. 



FIG. 554. Permanent teeth. Right side (Burchanl). 



The roots of the teeth are firmly implanted within the alveoli (Fig. 558) ; these 

 depressions are lined with periosteum (the pericementum) which is reflected on to 

 the tooth at the point of the root and covers it as far as the neck. At the margin 

 of the alveolus the periosteum becomes continuous with the fibrous structure of 

 the gums. 



Permanent Teeth (Figs. 555 and 556). 



The incisors, or cutting teeth, are so named from their presenting a sharp cut- 

 ting edge, adapted for incising the food. They are eight in number, and comprise 

 the four front teeth in each jaw. 



