PERMANENT TEETH. 1115 



in the molar and premolar teeth, and in the incisors also there are similar slight 

 prolongations of the cavity towards the angles of the crown. 



The roots of the teeth are embedded in the alveoli or sockets of the jaws, to 

 which they are accurately adapted, and firmly united (Fig. 880) by a highly 

 vascular layer of connective tissue the periosteum alveolare. This is attached 

 to the wall of the alveolus on the one hand and to the root of the tooth on the 

 other, whilst above it is continuous with the connective tissues of the gum. 



So accurately are the root and the alveolus adapted to each other over their 

 whole extent, and so firmly does the periosteum bind them together, that, under 

 normal conditions, the tooth is quite firmly fixed in the bone, and no movement 

 of the root within the alveolus can take place ; the vessels and nerves entering at 

 the apex are thus secured against pressure or strain. 



When, however, the alveolar periosteum is inflamed it becomes swollen and exquisitely sensi- 

 tive ; the tooth, as a result of the swelling, is pushed partly out of its socket, its crown projects 

 above those of its neighbours, and strikes against the opposing tooth when the mouth is closed, 

 giving rise to much pain and discomfort. 



The neck, although the term is useful, can scarcely be recognised as a distinct constriction in the 

 permanent teeth ; it corresponds to the line along which the gum and alveolar periosteum meet, 

 or along which the gum is united to the tooth ; but, as already pointed out, the gum does not 

 stop at the neck, but forms a free fold which surrounds the base of the crown collar-wise for a 

 short distance. The outline of the margin of the gum opposite the labial and lingual surfaces of 

 the crown is usually concave, but opposite the contact surfaces of the tooth it is convex, and 

 reaches much nearer to the edge of the crown than on the other surfaces. 



In the incisors and canines the tooth, cavity, which is about ^ to j the'diameter of the tooth, 

 passes very gradually into the root-canal (Fig. 880), so that it is difficult to say where one ends 

 and the other begins. The reverse is the case in the molars, whilst the premolars are somewhat 

 variable in this respect. 



Tartar is a hard calcareous deposit from the saliva (salivary calculus), often found on the teeth 

 near their necks. It is composed of lime salts, and its deposit is largely determined by the 

 presence of organisms (leptothrix, etc.) in the mouth. 



DENTES PERMANENTES. 



The permanent teeth (Figs. 881 and 882) are thirty-two in number, sixteen 



above and sixteen below, or eight in each half of both jaws ; and, although they 



can be grouped under four heads incisors, canines, premolars, and molars the 



individual teeth differ so much in their characters that each tooth requires a 



1 separate description. 



Descriptive Terms. Before describing the permanent teeth, it is requisite that 

 certain terms which are employed to denote the surfaces of the teeth should be defined. 

 This is a matter of some importance, seeing that the terms medial and lateral, anterior 

 and posterior, cannot, owing to the curvature of the dental arches, be properly applied 

 to all the teeth in the same sense. The terms given below have been adopted seeing that 

 they are free from the danger of misconception. 



The part of a tooth which comes in contact with the teeth of the opposite jaw is known 

 as the facies masticatoria (grinding or masticating surface) (Fig. 883). The surface in 

 contact with or looking towards its predecessor in the row is known as the facies medialis 

 in incisors and canines, facies anterior in premolars and molars ; the opposite surface, 

 namely, that which looks towards its successor in the row, is known as the facies lateralis 

 in incisors and canines, facies posterior in molars and premolars. The surface which 

 looks towards the tongue is the facies lingualis (lingual surface), and that looking in 

 the opposite direction, i.e. towards the lips and cheek, the facies labialis (labial surface). 

 The portion of a tooth which touches its neighbour in the same row is known as the 

 facies contacta (contact surface). 



Dentes Incisivi (Figs. 881 and 882). The incisor teeth, four in number in 

 each jaw, are used specially for cutting the food, hence their name. The crown 

 of each is chisel-shaped, and presents a labial surface which is convex in all 

 directions, a concave lingual surface, and a chisel-like edge, which, when first 

 cut, is surmounted by three small tubercles separated by two grooves. These 

 tubercles, however, are soon worn down, and the edge becomes straight or nearly 

 so. Owing to the fact that the upper incisors overlap those in the mandible, the 



