1114 



THE DIGESTIVE SYSTEM. 



cut between the seventeenth and twenty- fifth year, but are often delayed until a 

 very much later period, and occasionally never appear. 



The set of teeth which, as indicated above, begin to appear in the infant about 

 the sixth month, are known as the deciduous teeth (O.T. temporary, or milk teeth) ; 

 whilst those which succeed them and form the adult equipment are the permanent 

 teeth. 



The deciduous teeth are twenty in number, and are named as follows in each jaw, 

 beginning at the median plane : dentes incisivi, or incisor teeth, central and lateral ; 

 dens caninus, or canine tooth ; dentes molares, or molar teeth, first and second ; or 

 more briefly, two incisors, one canine, two molars. This is conveniently expressed 

 by the " dental formula " for the deciduous teeth in man, which shows the number 



of each class of teeth above and below on 

 one side of the mouth, viz. : 



i. f , c. ^, ra. f = 20. 



The permanent teeth, thirty -two in 

 number, are named dentes incisivi, or in- 

 cisor teeth, central and lateral ; dentes 

 canini, or canine teeth ; dentes praemolares, 

 premolar (O.T. bicuspid) teeth; dentes 

 molares, molar teeth ; and are arranged 

 as follows in each jaw, beginning at the 

 median plane : central incisor, lateral in- 

 cisor, canine, 1st premolar, 2nd premolar, 

 1st molar, 2nd molar, and 3rd molar or 

 dens serotinus (O.T. wisdom tooth). The 

 dental formula for the permanent set in 

 man is thus : 



Crown 



Substantia 

 adamantina 



Snbstantia 

 eburnea 



Cavum 

 dentis 



Neck 



Root 



c. 



pm. I, ra. |-32. 



Bone 



Substantia ossea 



Alveolar periosteum or root-membrane 

 FIG. 880. VERTICAL SECTION OF CANINE TOOTH, 

 to illustrate its various parts, and its structure. 



General Form and Structure. A 



tooth consists (Fig. 880) of (1) the corona 

 dentis or crown, the portion projecting 

 above the gum. It varies in shape in the 

 different teeth, and in all, except th< 

 incisors and canines, bears on its masticating 

 surface a number of tubercles, the tuberci 

 coronae (O.T. cusps), varying in number froi 

 two to five in the different teeth ; (2) th( 

 collum dentis or neck, the faintly constricte< 

 part which is surrounded collar- wise by the 

 gum, and connects the crown with (3) 

 the radix dentis or root, the portion of 

 the tooth embedded in the alveolus of the jaw. In the majority of teeth, 

 namely, in all except the molars, the root, as a rule, is single, or nearly so, and 

 consists of a long, tapering, conical, or flattened piece, perfectly adapted to the 

 alveolus in which it lies. In the molar teeth (and in some of the others occasionally) 

 the root is divided into two or three tapering or flattened roots or fangs. At the 

 apex of each root there can be made out, even with the naked eye, a minute 

 opening, the foramen apicis, through which the vessels and nerves enter the tooth. 



When a section of a tooth is made (Fig. 880), it will be seen that the interior 

 of the body is occupied by a cavity of some size, called the cavum dentis or 

 tooth cavity, which is filled in the natural state by the soft and sensitive 

 tissue known as the pulpa dentis or tooth pulp. The tooth cavity gradually narrows, 

 and is prolonged into each root of the tooth as a slender tapering passage, the 

 canalis radicis (root-canal), which opens at the apical foramen already referred 

 to. Through these root -canals, which also contain some pulp, the vessels and 

 nerves, which enter at the apex, pass to the interior of the tooth. 



Short diverticula of the pulp cavity are prolonged into the bases of the tuberch 



