124

Chemical Composition of Teeth.

ting edge or grinding surface of the crown, and vanishing upon the neck of the tooth.

If a tooth be steeped in diluted muriatic acid, it retains its form, but becomes flexible; the acid dissolves the earthy matter, and leaves the animal substance with which it was combined.

The following is the composition of the bone of teeth:

If a section be made through a tooth and the alveolar process which contains it after a successful injection, neither the enamel nor bony part appears in any degree reddened, but a fine vascular membrane, which enters at the aperture of the fang, is seen to line the whole of the cavity of the tooth; branches from the fifth pair of nerves may be traced to the opening of the fang, upon which the sensibility of a tooth depends.

In this manner the teeth cohere with neighbouring vascular parts. Their mode of life and growth will be

Number and Shape of Human Teeth.

125

afterwards described. At present we have only to consider their mechanical agency in comminuting the food.

The four front teeth in each jaw are termed incisors: the crown has a cutting edge extending transversely, and is wedge-shaped: the fang is single: the two central incisors in the upper jaw are longer than the rest, so as to throw the remaining teeth of the upper jaw rather without or behind those of the lower, till the smallness of the last grinder in the upper jaw causes them to terminate at the same vertical plane.

A cuspidatus, canine, or eye-tooth is found next to the incisors; it is pointed, and larger than the preceding : its fang is single but of great length, and frequently bent at the extremity.

The two sets of cuspidati and incisors form two curved blades, which meet like those of scissors; the incisors and cuspidati of the upper jaw generally fall before those of the lower.

The two teeth, which immediately follow each cuspidatus, have two points upon the crown, one without the other, and the largest external; they are called bicuspides: they have one broad fang fluted at its sides.

The three remaining teeth on each side of each jaw are called molares or grinding teeth: their crowns have five points. The molares of the lower jaw have two fangs, one behind the other; of the upper, three, two of which are external; they rise in a slanting direction. The distribution of the fangs of the upper molares is intended to avoid the antrum of Highmore; but it often happens that one fang, and that more generally belonging to the second molaris, extends into the antrum. In the museum of Albinus there is an instance of the crown of a molar tooth

126 Form of Teeth characteristic of the Habits of Animals.

growing into the cavity of the antrum, the direction of the fangs being reversed. The first molaris is the largest; the posterior, or dens sapientiæ, is small: its fangs grow together, and are short. The grinding surfaces of the two sets of molar teeth are exactly opposed to each other. The strictest relation exists between the form of the teeth and the habits of animals. Thus in the horse, which crops the herbage by bruising and snapping it across, the incisors have broad cutting edges, which meet like the blades of pincers: in the incisor teeth of the beaver, which gnaws through the hardest vegetable fibre, a sharp edge is preserved by the disproportionate distribution of enamel upon the fore part: in the lion the incisors are pointed: in the elephant, front teeth grow from the upper jaw only, and are prolonged into tusks, by the aid of which, with its trunk, the animal tears up the plants that serve for its food. The cuspidati are remarkably large in carnivorous quadrupeds which seize and rend their prey when living; and the character of the head is determined by the prominence of the zygoma to give room for the thick temporal muscle by which the jaws are closed, and by the shortness of the jaws which saves expenditure of power in closing them.

The molares are best developed in graminivorous animals; on this occasion a third substance termed the crusta petrosa, having less hardness than the bone, as the bone has less than the enamel, is wrought into their composition; and as each of these three substances is exposed upon the grinding surface, the latter derives a permanent inequality from their different degrees of hardness favourable to the comminution of the food. form of the heads of graminivorous quadrupeds is charac

The

1

Movements of the lower Jaw.

127

terized by the length of the jaws in which the massive grinding teeth are set; by the long and flat zygoma, and by the depth and breadth of the branches of the lower jaw, to which muscles are attached, which move it forward and laterally.

The lower jaw consists of the curved piece of bone, in which the sixteen teeth are set, and of a process or branch on either side, which rises nearly at a right angle to be articulated by means of its condyle with the glenoïd fossa of the temporal bone.. Each branch is of such a length that when the lower jaw is fully raised, the two rows of teeth are equally pressed against each other, the front teeth locking, the molar teeth simply meeting.

The elementary motions of the lower jaw consist in its simple elevation or depression, in its horizontal movement forward or backward, and from side to side.

1. During the depression or elevation of the lower jaw, the centre of motion falls about the middle of its branches. Or the lower jaw in rising or falling performs part of a vertical revolution, upon an imaginary line drawn horizontally across from side to side through the middle of its branches. The angle of the jaw is carried upwards and backwards; the condyle forwards and downwards, sliding upon the interarticular cartilage, which separates it from the os temporis. The temporal muscles directly raise the lower jaw: the digastricus and other muscles which depress it at the same time retract it; and thus admit of being brought into play even during the action of raising the jaw, in order to limit the effect of the masseter and internal pterygoïd muscles which tend to carry the jaw forward as well as upward.

2. The lower jaw may be carried forward in a plane

128 Of the muscular Structure of the Tongue.

parallel to that of the alveoli of the upper jaw, by the action of the external pterygoïd muscles,-aided by the masseters and internal pterygoïd muscles, if their tendency to raise the lower jaw be prevented by the digastrici and various other muscles attached to the os hyoïdes. The muscles last alluded to are calculated simply to retract the jaw, when their effect towards its depression is neutralized by the temporal, masseter, and internal pterygoïd muscles.

3. The lower jaw may rotate horizontally round an imaginary centre, which falls in the middle of a right line joining the two condyles: the masseter of the same side and the pterygoïdei of the opposite concur in giving the jaw this movement with the digastrici and various other muscles attached to the os hyoïdes, the action of which preserves the movement horizontal.

By differently combining these simple motions, all the variety of pressure which the teeth make upon the food is produced.

The os hyoïdes is composed of three slight pieces of bone, a base and two cornua, forming a small horse-shoe figure, within and behind the more capacious curve of the lower jaw upon this bone the mass of flesh which forms the tongue is supported. The central and largest muscle of the tongue is termed the genio-hyo-glossus; it extends from the symphysis of the jaw to the os hyoïdes in one direction, to the tip of the tongue in the other. Other muscles slant backwards from the lower jaw to the os hyoïdes, which, with the preceding, raise the os hyoïdes, and carry the tongue forward or laterally: the linguales shorten the tongue, the stylo-glossi give it breadth and concavity, the hyo-glossi render it convex: