a = Eee eee 965 
At the end of this period, namely, about the sixth year, the next stage in the 
production of the adult condition begins. It consists in the eruption of four new 
teeth—the first permanent molars—one on each side, above and below, behind those 
of the milk set. This is followed by the gradual falling out of the twenty teeth 
which have occupied the mouth since the second year (Fig. 648), and the sub- 
stitution for them of twenty new teeth, which take up, one by one, the vacancies 
created by the dropping out of each of the milk set. Finally, “the adult condition 
is attained by the eruption of eight additional teeth—the Ind and 3rd molars— 
two on each side, above and below, behind those which have already appeared. All 
of these—the permanent set—have appeared by the end of the twelfth or thirteenth 
year, except the four wisdom teeth, which are usually 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, temporary, or milk teeth (dentes 
decidui), whilst those which succeed them and form the adult equipment are the 
permanent teeth (lentes permanentes). 
The milk teeth are twenty in number, and are named as follows in each jaw, 
beginning at the middle line :—central incisor, lateral incisor, canine, first molar 
and second molar; or more briefly, two incisors, one canine, two molt This is con- 
veniently 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.:— 
Tae ae Enamel 
The permanent teeth, thirty-two in 
number, are named in each jaw, beginning 
at the middle lne:—central incisor, lateral Ctow” 
incisor, canine, Ist premolar (or bicuspid), 
2nd premolar (or bicuspid), Ist molar, 2nd 
wolar, and 3rd molar or wisdom tooth ~ 
(dens serotinus). The dental formula for 
the permanent set in man is thus :— 
Dentine 
DP 
Yy 
a. 2, ¢e. 2, pm. 2, m. 3. 
Q 1? L a0 a Neck 
General Form and Structure.—A tooth 
consists (Fig. 649) of (1) the crown (corona 
dentis), the portion projecting above the 
gum, which varies in shape in the different 
teeth, and in all, except the incisors and 
canines, bears on its head or grinding surface 
a number of tubercles or cusps (tul fer@ulay ae, 
coron), varying in number from two to five 
in the different teeth; (2) the neck (collum 
dentis), the faintly constricted part which is 
surrounded collar-wise by the gum, and 
which connects the crown with (3) the root 
(radix dentis), the portion of the tooth 
which is embedded in the alveolus of the 
maxilla or the mandible. In the majority Gomer’ an eruste petross 
of teeth, namely, in all except the molars, Alveolar periost eum or root-membrane 
the root, as a rule, 1s single, OF nearly SO, Fic. 649.—Verticar SEcTION OF CaNINE TOOTH, 
and consists of a long, tapering, conical, OY to illustrate its various parts, and its structure. 
flattened piece, perfectly adapted to the 
alveolus in which it lies. In the molar teeth (and in some of the others occasion - 
ally) 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 (foramen apicis) through which the vessels and nerves enter the 
tooth. 
IIIA LIED: 
2 eee 
