In the human adult, the teeth are composed of two substances ; a bony matter differing in some respects from the other bony parts of the system, forming the fangs and central part of the body of the tooth ; and a much harder white, shining substance, covering the outer part of the body, usually called enamel, and lately denominated by Dr Blake, corner striatus, or striated portion, from its appearance when minutely examined.
The bony part is seldom perfectly solid, hut contains within it a central cavity, having nearly the form of the tooth, and filled with a soft vascular substance. These cavities decrease as the person advances in life, and are at last nearly obliterated. Their appearance is represent ed at a, Fig. 6. and 7, Plate XIX. The enamel, or stri ated portion of the teeth, invests the bony part only in the body of the tooth, and is thickest towards the cut ting or grinding surface. This portion is composed of fibres standing nearly parallel to each other, and per pendicular to the body of the tooth (see Fig. 6. and 7, c.) ; structure which may easily be seen with a magnifying glass in a broken tooth. This enamel is extremely hard, and is not capable, like the bony part of the tooth, of re ceiving a tinge from absorption of the particles of mad der.
Towards the fangs, where the enamel ceases, there is a remarkable change in the appearance of the bony part, which is here semi-transparent, resembling horn. That the several parts composing the teeth differ both from each other, and from the ordinary bones that form the skeleton, is proved, as well from their want of that reticulated texture which distinguishes the latter, as from the different proportions of their chemical constituents.
See CHEMISTRY.
Though the gums are among the most insensible parts of the body, so that they may be cut, or even torn with impunity, the teeth are, as is well known to the martyrs of toothach, extremely sensible. This exquisite sensi bility seems to reside wholly in the internal pulp, or the vascular membrane that lines the cavity of the tooth. The nerves that supply the teeth are derived from the maxillary nerves, and both these and the corresponding arteries send a branch through each fang as they pass by the sockets.
It is well known, that an infant at birth is almost al ways without teeth, and that a few months after birth the front teeth begin to make their appearance through the gum. The gradual formation of the teeth within the jaws has not, till lately, been fully understood. Repeat
ed obser'iation has shown, that the ossification of the jaws begins to take place in the foAus about two months after conception, before which time these parts are en tirely composed of membrane and cartilage. By de grees the which, for a long time, were com posed of at least two pieces, begin to unite towards the chin ; two bony plates are formed, one next the cheeks, and the other next the cavity of the mouth, leaving be tween them a groove, or channel, containing vessels, nerves, and little soft, pulpy bodies, which are the rudi ments of the teeth, and which are separated from each other by peculiar membranes. As the ossification of the jaws advances, bony fibres shoot across from plate to plate, gradually forming an inclosure around each pulpy body, and constituting the future sockets of the first teeth. Usually, between the eighth and ninth months after conception, bony fibres begin to appear in the pulpy rudiments of the teeth, and at birth a bony shell is form ed round each, surrounded by a peculiar membrane, which is thickest next the gum. As this bony shell increases, the central pulp diminishes ; but, from being at first nearly of a spherical figure, it is gradually length ened out to form the central part of the future fangs. As the ossification proceeds, the tooth gradually rises in the socket, and advances with its investing membrane towards the surface of the gum. Now, the striated por tion begins to be formed, and appears to be secreted from the investing membrane of the bony shell ; as this membrane disappears, in proportion as the enamel is produced. The front teeth generally begin to make their appearance about the sixth, seventh, or eighth month, after birth ; and those first cut are generally the middle incisors of the lower jaw ; about the twelfth or fourteenth month, the bicuspidated teeth, or small grind ers of the lower jaw, begin to make their appt."7.nce, and, about the twentieth month, the cuspidated or eye teeth. These periods vary considerably in different children, some beginning to cut their teeth by the fourth of fifth month, while others remain without any teeth for more than two years; but, in most instances, by the time the child has attained its third year, it has acquired 20 teeth, viz. all the front or cutting teeth, the four eye teeth, the four bicuspidated teeth, and the first grinders on each side.