Through this fatty cellular membrane, runs the ex cretory duct of the parotid gland, opening within the mouth at a perforation in tire buccinator muscle, oppo site to the second or third grinder of the upper jaw.
There are dine glandular bodies on each side which secrete the saliva that is mixed with the food during mastication, viz. the parotid gland, situated in the Fore part of the car, and between that organ and the angle of the lower jaw, partly covering the masseter muscle ; the submaxillary gland, lying on the inside of the angle of the lower Jaw, and covering the tendon of the maxilla', and the subl tigual gland, lying below the fore part of the tongue. The structure of these glands will he more properly considered when we come to describe the organs or secretion.
All these organs, and the whole interior of the mouth, arc lined with an extensive mucous membrane, that, from its numerous fold and connections, deserves particular notice. We shall trace its connections, suppo sing it to continence at some particular part, and in doing this, we shall chiefly follow Bichat, who commences with the border of the lower lip. After investing the hack part of the lower lip, this mucous membrane is reflect ed over the body of the jaw-bone, at the symphysis of which it forms a remarkable doubling, called the ire num of the lip. It then ascends towards the necks of the teeth, and gives off to each socket a promulgation, which, with the periosteum of the jaw, contributes to form the gums. Passing between the teeth, it lines the inner surface of the lower jaw, and at the inner symphy sis of the jaw forms the helium of the tongue. Then passing over the sublingual glands, and covering the in ferior surface of the tongue, as far as its point, it advan ces over this and the edges of the tongue, covers its up per surface as far as its base, and unites with the mem brane that covers the epiglottis. This is the course which Bichat supposes the mucous membrane to pursue in the inferior or basilar portion of the mouth ; and we May suppose it to follow a similar route from the edge of the upper lip, and its anterior surface, where it forms a similar frenum over the alveolar processes of the up per jaw-bones, where it also contributes to form the gums, then lining the palatine arch, and the inner surface of the -velum pendulum, over the lower border of which it is reflected, and meets the pituitary membrane that comes behind from the nostrils. Again, we trace it from the commissures of the lips, lining the inside of the cheeks, and proceeding downward, covers the lateral borders and pillars of the velum pendulum, including the tonsils, and uniting with the portion which we have just before traced. Throughout the whole extent of this mucous membrane there are numerous glands contained within its doublings, and its surface next the cavity of the mouth is lined with a very delicate epidermis, or, as it is called by anatomists, epithelium.
The principal organs of mastication are, as we have said, the teeth, the structure and developement of which are among the most curious enquiries, that can the attention of the anatomist and physiologist, and have accordingly been examined by both with great minute ness.
The number of teeth in a full-grown man is usually 32, 16 in each jaw. These are divided into classes, ac
cording to their form or uses. The four front teeth in each jaw are called incisores incisivi, or cutting teeth ; the single teeth that stand next these on each side are called caning, or cuspidati, dog or eye teeth ; next these stand two teeth on each side, which from their ending in two projecting points, have been called bicusjddes, or two pointed teeth. They are also called small grinders. These are most distinct in the upper jaw, as in the lower jaw the points, and especially those next the tongue, are much less prominent. The three teeth on each side of these last, have three, four, or five project ing points, and are called molares, or grinders.
Each of these teeth is said to consist of three parts ; a body, which is that part without the sockets ; roots or fangs, by which they are fixed in the sockets ; and a neck between these, which is surrounded by the gums. The classes of teeth we have enumerated differ from each other, both in the form of their bodies, and in the form and number of their fangs. (Plate XIX. Fig. 1, 2.) The bodies of the cutting teeth are flat anti broad, rounded on the fore part, and generally a little concave internally, and end in a sharp or cutting edge, is sometimes a little serrated. These teeth halve each a single fang, which is usually flattened at the sides. The bodies of the canine teeth arc nearly wedge-shaped, but their extremities, especially in the upper jaw, are more pointed than those of the cutting teeth, and the fangs are larger, thicker, and more flattened on the sides than those of the former, and sink much deeper in the sock ets. The bicuspidated teeth, beside differing from the two former in their pointed extremities, have each a large double fang, which, from a depression on each side, appears like two fangs united ; and indeed some times, especially in the upper jaw, these teeth have two distinct fangs. The bodies of the grinders are large and thick, and a horrizontal section of them appears, nearly square. (Fig. 3, 4, and 5.) They differ much, and pretty regularly, in the number of their points. The grinder next the bicuspidated teeth on each side in the upper jaw, has usually four points, and the rest in that jaw have seldom more than three ; while the first grind er in the under jaw has generally five, and the others four points. Most of the grinders have two, and some of them three fangs, passing through separate orifices in the jaw, and spreading widely from each other they enter. These fangs are flattened from before back wards, and not laterally, as in the other teeth. The last or most backward grinders are often called dentc.s sapientix, or wisdom teeth, as they are seldom cut be fore the age of puberty, when most young people arc supposed to attain the years of discretion. All the roots or fangs of the teeth are of a conical form, broad est next the body of the tooth, and gradually tapering to a point. Each fang is perforated with a canal for the passage of blood-vessels and nerves, from the socket to the body of the tooth ; and each fang is invested with a periosteum or membrane, derived from the inner sides of the sockets.