966 THE DIGESTIVE SYSTEM. 
On making a section of a tooth (Fig. 649), it will be seen that the interior of 
the body is occupied by a cavity of some size, generally called the pulp cavity 
(cavum dentis), owing to the fact that it is filled in the natural state by the soft 
and sensitive tissue known as the pulp. This pulp cavity gradually narrows 
below, and is prolonged into each root of the tooth as a slender tapering passage, 
the root canal (canalis radicis), which opens at the apical foramen already referred 
to. Through these root-ca nals, 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 cusps in 
the molar and premolar teeth, and in the incisors also there are similar slight pro- 
longations of the cavity towards the angles of the crown. 
The roots of the teeth are embedded in the sockets or alveoli of the jaws, to 
which they are accurately adapted, and firmly united (Fig. 649) by a highly 
vascular layer of connective tissue alveolar prea ( ‘alveolo-dental perlos- 
teum or root-membrane). This is Bhiaened 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 a useful term, 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 proximal and distal sides 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 pulp cavity, which is about } to } the diameter of the tooth, 
passes very gradually into the root canal (Fig. 649), so that it 1s ditticult 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, ete.) in the mouth. 
THE PERMANENT TEETH. 
The permanent teeth (Figs. 650 and 655) are thirty-two in number, sixteen above 
and sixteen below, or eight in each half of either jaw; and, although we can group 
them under four heads—incisors, canines, premolars, and molars—the individual 
teeth differ so much in their characters that each tooth will require a separate 
deseription. 
Descriptive Terms.—Before describing the permanent teeth, it will be well to 
explain certain terms used to denote the surfaces of the teeth, a matter of some import- 
ance, seeing that the terms inner and outer, anterior and posterior, cannot, owing to the 
curvature of the dental arches, be properly applied to all the teeth in the same sense. 
For this reason the terms given below have been adopted as being free from misconception. 
The part of a tooth which comes in contact with the teeth of the opposite jaw is known 
as the grinding or masticating surface (facies masticatoria, Fig. 652). The surface in 
contact with or looking towards its predecessor in the row is known as the proximal 
surface (facies medialis in incisors and canines, factes anterior in premolars and molars) ; 
the opposite surface, namely, that which looks towards its successor in the row, is known 
as the distal surface (facves /atervalis in incisors and canines, facies posterior in molars and 
premolars). The surface which looks towards the tongue is the lingual surface (facies 
lingualis), and that looking in the opposite direction, ¢.e. towards the lips and cheek, the 
labial surface (facies labialis). The portion of a tooth which touches its neighbour in the 
same row is known as the contact surface (facies contactus). 
