The number of the teeth is sixteen in each jaw. These are divided into classes, according to their shape and use. There are, in each jaw, four incisores; two cuspidati or canine teeth; four bicuspidati; and six molares or grinders. Each tooth has three parts: the crown, neck, and fang or root; the first being the part above the gum; the second that embraced by the gum; and the third, the part contained in the alveolus or socket. The crown varies in the different classes. In the incisors, it is wedge-shaped; in the canine, conical; and in the molar, cubical. In all, it is of extreme hardness, but in time wears away by the constant friction to which it is exposed. The incisor and canine teeth have only one root; the molares of the lower jaw, two; and the upper, three. In all cases, they are of a conical shape, the base of the cone corresponding to the corona, and the apex to the bottom of the alveolus. The alveolar margin of the jaws is covered by a thick, fibrous, resisting substance, called gum. It surrounds accurately the inferior part of the crown of the tooth, adheres to it strongly, and thus adds to the solidity of the junction of the teeth with the jaws. It is capable of sustaining considerable pressure without inconvenience. But we shall have to return to the subject of the teeth hereafter. I

The articulation of the lower jaw is of such a nature as to admit of depression and elevation; of horizontal motion forwards, backwards, and laterally; and of a semi-rotation upon one of its condyles. The muscles that move it may be thrown into two classes: elevators and depressors. These, by a combination of their contraction, can produce every intermediate movement between elevation and depression. The raisers or levator muscles of the jaw extend from the cranium and upper jaw to the lower. They are four in number on each side, the temporal,

Fig. 211

and masseter, which are entirely concerned in the function; the external pterygoid, which, whilst it raises the jaw, carries it at the same time forward, and to one side; and the internal pterygoid, which, according as it unites its action with the temporal or with the external pterygoid, is an elevator of the jaw or a lateral motor. The depressors may be divided into immediate and mediate, according as they are, or are not, attached to the lower jaw itself. There are only three of the former class: 1, the digastricus, the anterior fasciculus of which, or that which passes from the os hyoides to the lower jaw, depresses the latter; 2, the geniohyoideus; and 3, the mylo-hyoideus, all of which concur in the formation of the floor of the mouth. The indirect or mediate depressors are all those, that are situate between the trunk and the lower jaw, without being directly attached to the latter;-as the thyro-hyoideus,

Skull of the Polar Bear.

the sterno-thyroideus, and the omo-hyoideus; the names of which indicate their origin and insertion. These, in the aggregate, form a muscular chain, which, when it makes the trunk its fixed point, depresses the lower jaw. The arrangement of the elevators and depressors is such, that the former predominate over the latter; and hence during sleep the jaws continue applied to each other, and the mouth is consequently closed.

The human organs of mastication hold an intermediate place between those of the carnivorous and herbivorous animal. In the carnivorous animal, which has to seize hold of, and retain its prey between its teeth, the jaws have considerable strength; and the movement of elevation is all that is practicable; or, at least, that can be effected to any extent. This is dependent upon organization. The condyle is broader from side to side, which prevents motion in that direction: the glenoid cavity is very deep, so that the head of the jaw-bone cannot pass out of it; and it is, moreover, fixed in its place by two eminences before and behind. The muscular apparatus is also so arranged as to admit of energetic action on the part of the muscles that raise the jaw; but of scarcely any in a horizontal direction. The deep impressions in the regions of the temporal and masseter muscles indicate the large size of these muscles in the purely carnivorous animal; whilst the pterygoid muscles are extremely small. The teeth, too, are characteristic; the molars being comparatively small, at the same time that they are much more pointed. On the other hand, the cuspidati are remarkably large, and the incisors, in general, acuminated.

The herbivorous animal has an arrangement the reverse of this. The condyle or head of the lower jaw is rounded; and can, therefore, be moved in all directions; and as easily horizontally as up and down. The glenoid cavity is shallow, and yields the same facilities. The articulation, which is very close in the carnivorous animal, is here quite loose. The levator muscles are much more feeble; the temporal fossa is less deep; the zygomatic arch less convex; and the zygomatic fossa less extensive. On the other hand, the pterygoid fossa is ample and the muscles of the same name are largely developed. The molares are large and broad; and their magnitude is so great as to require, that the jaw should be much elongated in order to make room for them.

The joint of the lower jaw has, in man, solidity enough for the jaws to exert considerable pressure with impunity, and laxity enough that the lower jaw may execute horizontal movements. The action of the leva

tor muscles is the most extensive; but the lateral or grinding motion is practicable to the necessary extent; and the muscles of both kinds have a medium degree of developement. The teeth, likewise, partake of the characteristics of those of the carnivorous and herbivorous animals;-twelve-the canine teeth and lesser molares-corresponding to those of the carnivorous; and twenty-the incisors and larger molares to those of the herbivorous.

The tongue must be regarded as an organ of mastication. It rests horizontally on the floor of the mouth; is free above, anteriorly; and, to a certain extent, beneath and at the sides. Behind, it is united to the epiglottis by three folds of the mucous membrane of the mouth; and is supported at its base by the os hyoides, with which it participates in its movements. The tongue, as the organ of taste and articulation, has been described already (p. 145). We have only, therefore, to describe the os hyoides and its attachment to that bone. The hyoid bone has, as its name imports, the shape of the Greek letter v, the convex part being before. (Fig. 194.) It is situate between the tongue and larynx and is divided into body or central part; and into branches, one extremity of which is united to the body by an intermediate cartilage, that admits of slight motion; whilst the other is free, and is called greater cornu. Above the point, at which the branch is articulated with the body, is an apophysis or process, called lesser cornu. The os hyoides is united to the neighbouring parts by fibrous organs, and muscles. The former are;-above, the stylo-hyoid ligament, which extends from the lesser cornu of the bone to the styloid process of the temporal bone; below, a fibrous membrane, called thyro-hyoid, passing between the body of the bone and the thyroid cartilage; and two ligaments, extending from the greater cornu of the hyoid bone to the thyroid cartilage, called thyro-hyoid. Of the muscles; some are above the hyoid bone, and raise it;—viz., the genio- and mylo-hyoideus, already referred to; the stylo-hyoid, and some fibres of the middle constrictor of the pharynx. Others are below, and depress it. They are the sterno-thyrohyoideus, omo-hyoideus and sterno-thyroideus. The base of the tongue is attached to the body of the bone by a ligamentous tissue, and by the fibres of the hyoglossus muscle.

Among the collateral organs of mastication are those which secrete the saliva, and the various fluids which are poured out into the mouth, -constituting together what has been termed the apparatus of insalivation. These fluids proceed from different sources. The mucous membrane of the mouth, like other mucous membranes, exhales a serous or albuminous fluid, besides a mucous fluid secreted by the numerous follicles contained in its substance. Four glands likewise exist on each side, destined to secrete the saliva, which is poured into the mouth by distinct excretory ducts. They are the parotid, submaxil lary, sublingual, and intra-lingual or lingual. The first is situate between the ear and the jaw; and its excretory duct opens into the mouth opposite the second small molaris of the upper jaw. By pressing upon this part of the cheek, the saliva can be made to issue into the mouth, in perceptibly increased quantity. The submaxillary gland is situate beneath the base of the jaw; and its excretory duct opens into

Fig. 213.

the mouth at the side of the frænum linguæ. The sublingual gland is situate under the tongue, and its excretory ducts open at the sides of that organ, and the intra-lingual or lingual is seated at the inferior surface of the tongue, where the membrane

forms a fringed fold. These glands are constantly pouring saliva into the mouth; and it has been presumed, that the fluids secreted by them may differ from each other in physical and chemical characters. Such, at least, has been the view of some as regards the sublingual, the texture of which more nearly resembles that of the compound follicles than of glands; but the circumstance has not been proved by any direct experiment. The saliva, as met with, is a compound of every secretion poured into the mouth; and it is this fluid which has been chiefly subjected to analysis. The secretion of the saliva, and its various properties, will be considered, however, hereafter.

1. Parotid gland in situ, extending from the zygoma above, to the angle of the jaw below. 2. Duct of Steno. 3. Submaxillary gland. 4. Its duct. 5. Sublingual gland.

The two apertures of the mouth are the labial and pharyngeal. The former, as its name imports, is formed by the lips, which consist externally of a layer of skin; are lined internally by a mucous membrane; and, in their substance, contain numerous muscles, already described under the head of Gestures. These muscles may be separated into constrictors and dilators; the orbicularis oris being the only one of the first class, and the antagonist to the others, which are eight in number, on each side-levator labii superioris alæque nasi, levator labii superioris proprius, levator anguli oris, zygomaticus major, zygomaticus minor, buccinator, triangularis, and quadratus menti. (Fig. 201.) To the last two muscles are added some fibres of the platysma myoides. The pharyngeal opening is smaller than the labial, and of a quadrilateral shape. It is bounded above by the velum palati or pendulous veil of the palate; below, by the base of the tongue; and laterally, by two muscles, which form the pillars of the fauces. The pendulous veil is a musculo-membranous extension, constituting a kind of valve, attached to the posterior margin of the bony palate, by which all communication between the mouth and pharynx, or between the pharynx and nose can be prevented. (Fig. 214.) To produce the first of these effects, it becomes vertical; to produce the latter, horizontal. At its inferior and free margin, it has a nipple-like shape, and bears the name

Cavity of the Month, as shown by dividing the An

gles and turning off the Lips.

1. Upper lip, turned up. 2. Its frænum. 3. Lower lip, turned down. 4. Its frænum. 5. Internal surface

of cheeks. 6. Opening of duct of Steno. 7. Roof of mouth. 8. Anterior portion of lateral half arches. 9. or palato-staphylinus.

Posterior portion of lateral half arches. 10. Velum pendulum palati. 11. Tonsils. 12. Tongue.