DENTISTRY.
There are 32 permanent teeth in man, 16 in the upper and 16 in the lower jaw; they are also arranged in symmetrical sets of 8 teeth on each side. The upper and lower symmetrical sets of 8 will be described. The two teeth from the mid-line in front are "incisors" and have chisel-shaped crowns. The central incisor of the upper jaw is broader than any of the others, and bites against the central and lateral incisors of the lower jaw; the same want of exact, adaptation continues throughout the series, so that every tooth in the upper jaw except the last molar bites against its corresponding tooth of the lower jaw and also against the tooth behind that.
Next to the incisors comes the "canine (or eye) tooth," the crown of which is somewhat peg-shaped, while behind this are the two "premolars" or "bicuspids," whose crowns are flattened from before backward and bear two cusps, the larger of which is external. As a rule there is a single root, though sometimes in the first upper premolar it is double.
The three "molars" are placed behind the premolars, and the upper and lower sets can be easily distinguished because the upper have three roots while the lower have only two. The grind ing surface of the crowns of the upper molars shows three or four cusps, while on the grinding surface of the lower molars four or five cusps are found.
Of the three molars the first is the largest, and the third, or wisdom tooth, the smallest, while the upper wisdom tooth is smaller than the lower.
In the "milk teeth" or temporary dentition of the child there are only 20 teeth, 10 in each jaw and 5 in each segment. They are two incisors, one canine and two so-called molars. These molars occupy the position which the permanent premolars later on take. The temporary teeth differ from the permanent in their smaller size, their whiter colour, the greater constriction of their necks, and in the fact that the roots of the molars are widely splayed.
The dates at which the milk teeth are cut are very variable. The lower central incisors come first between the sixth and ninth month, or even later; then come the central and lateral upper incisors ; then the lower lateral incisors, followed closely by the first molars. After a rest of four or five months come the canines,
while by about the end of the second year the second molars have appeared, and the milk dentition is complete. The milk teeth are cut in batches with resting intervals between.
The first permanent tooth to be cut is the first molar, and this happens during or soon after the sixth year. It does not displace any of the milk teeth, but comes down behind the second milk molar. During the seventh year the central milk incisors fall out and their place is taken by the permanent ones; the shed teeth are mere shells of the crown, all the root having been absorbed.
The lateral incisors succeed their milk predecessors at about eight years old, the first premolar takes the place of the first temporary molar about nine, the second premolar that of the second temporary molar about ten, the canine about eleven, while the second molar comes down behind the first about twelve. The third molar, or wisdom tooth, usually appears between eighteen and twenty, but may be much later, indeed it is sometimes never cut at all, and when it is, it often does not come down to a level with the other teeth. It is believed that man is gradually under going a suppression of his last molar teeth.
Histology.—In a vertical section the exposed part or crown is seen to be covered with enamel, which, microscopically, is composed of fine hexagonal prisms arranged at right angles to the surface of the tooth, and formed chiefly of calcium phos phate with small amounts of cal cium carbonate, magnesium phos phate and calcium fluoride, but containing practically no organic matter. The enamel rests on the "dentine," of which hard yet elastic substance by far the greater part of the tooth is com posed. It is made of the same salts as the enamel, but contains in addition a good deal of organic matter and forms a structureless mass through which the fine "dentinal tubes" run from the pulp cavity to the periphery.