The last molars are known as the wisdom teeth, as they do not usually appear until the person has reached the “years of discretion.”  All animals that live on grass, hay, corn, and the cereals generally, have large grinding teeth, as the horse, ox, sheep, and elephant.

The following table shows the teeth in their order: 

        Mo.  Bi.  Ca.  In.  In.  Ca.  Bi.  Mo.

Upper  3   2   1   2   |   2   1   2   3  = 16
|                      } = 32
Lower  3   2   1   2   |   2   1   2   3  = 16

The vertical line indicates the middle of the jaw, and shows that on each side of each jaw there are eight teeth.

134.  Development of the Teeth.  The teeth just described are the permanent set, which succeeds the temporary or milk teeth.  The latter are twenty in number, ten in each jaw, of which the four in the middle are incisors.  The tooth beyond on each side is an eye tooth, and the next two on each side are bicuspids, or premolars.

The milk teeth appear during the first and second years, and last until about the sixth or seventh year, from which time until the twelfth or thirteenth year, they are gradually pushed out, one by one, by the permanent teeth.  The roots of the milk teeth are much smaller than those of the second set.

[Illustration:  Fig. 48.—­Temporary and Permanent Teeth together.

Temporary teeth:
  A, central incisors;
  B lateral incisors;
  C, canines;
  D, anterior molars;
  E, posterior molars

Permanent teeth:
  F, central incisors;
  H, lateral incisors;
  K, canines;
  L, first bicuspids;
  M, second biscuspids;
  N, first molars
]

The plan of a gradual succession of teeth is a beautiful provision of nature, permitting the jaws to increase in size, and preserving the relative position and regularity of the successive teeth.

[Illustration:  Fig. 49.—­Showing the Principal Organs of the Thorax and Abdomen in situ. (The principal muscles are seen on the left, and superficial veins on the right.)]

135.  Structure of the Teeth.  If we should saw a tooth down through its center we would find in the interior a cavity.  This is the pulp cavity, which is filled with the dental pulp, a delicate substance richly supplied with nerves and blood-vessels, which enter the tooth by small openings at the point of the root.  The teeth are thus nourished like other parts of the body.  The exposure of the delicate pulp to the air, due to the decay of the dentine, gives rise to the pain of toothache.

Surrounding the cavity on all sides is the hard substance known as the dentine, or tooth ivory.  Outside the dentine of the root is a substance closely resembling bone, called cement.  In fact, it is true bone, but lacks the Haversian canals.  The root is held in its socket by a dense fibrous membrane which surrounds the cement as the periosteum does bone.