Strueture.—The lower jaw is formed of two complete plates, united by cancellous tissue, which is traversed by a long curved canal (the inferior dental canal), which conveys the vessels and nerves that supply the teeth. This canal commences in a groove just above the superior dental foramen, which is situated on the internal surface of the ramus ; it then enters the substance of the bone, taking the course of the internal oblique line below, and parallel to which it runs as far as the second bicuspid tooth, where it divides into two canals, one short and wide, which terminates on the external surface of the bone at the inferior dental foramen ; and another smaller one, which continues onwards as far as the middle incisor tooth, where it ceases. From the upper side of this dental canal small tubes arise, which proceed to the alveoli ; they convey vessels and nerves to the fangs of the teeth. The situation and size of the dental canal vary according to the age of the individual. At birth it runs near the lower border of the bone, and is of considerable magnitude; after the second dentition it becomes placed just below the mylo-hyoid ridge ; in the edentulous jaw it runs along the alveolar border of the bone, its size is much diminished, and the mental foramen is found close upon the upper border of the bone.
Connexions and uses.—The lower jaw is arti culated with the temporal bones, and receives the sixteen inferior teeth. It gives attachment to fourteen pairs of muscles, viz. the temporal, the massetcr, the two pterygoids, the bucci nator, the superior constrictor of the pharynx, the depressor angiili oris, the depressor labii inferioris, the lcvator menti, theplatysina, the genio-hyo-glossus, the genio-hyoideus, the mylo-hyoideus, and the digastric. Four pairs of ligaments are attached to it, viz. the external and the internal lateral ligaments of the tem poro-maxillary articulation, the pterygo-maxil lary (or intermaxillary) ligament, and the stylo maxillary ligament. It forMs the lower part of the face and the cavity of the mouth ; it protects the tongue, salivary gland, and pharynx ; it differs from the upper jaw and from all the other bones of the head in its remarkable mobility ; and it contributes essentially to mastication as well as to deglutition and articulation.
Development.—The lower jaw at birth con sists of two lateral halves, which are united vertically in front along the median line by a piece of cartilage, forming what has been improperly called a symphysis. A few months after birth the removal of this cartilage coin mences, and the two halves of the bone become united below; hut not (infrequently a fissure remains above for several months. At this period the alveolar border is, like that of the upper jaw, very thick, and contains some large irregular cavities which lodge the first set of teeth. Besides the superior dental
foramen there is found in the foetus another, which leads to a temporary canal that supplies the first set of teeth, and behind the alveoli of the incisors may be observed a row of holes which are said to be connected with the de velopment of the second set of teeth. Some authors maintain that each side of the lower jaw is developed by four separate points of ossification ; but this assertion wants confirma tion. It is certain that this bone is among those which are the most early developed, and in the embryo of two months it is already of considerable size. Its alveolar border is at first a mere groove, of which the internal margin is defective, and which gradually be comes hollowed into separate sockets as the teeth are developed. The changes of form which the lower jaw undergoes from birth till old age depend chiefly upon the development and decay of the teeth. Some of these changes have been already noticed, and will be found to correspond with those which occur in the alveolar border of the upper maxilla ; the varying form and direction of the rami and angles of the lower jaw we have noticed, and for the more detailed account of the de velopment of this bone as connected with dentition, we refer to the article TEETH.
Of the face in general.—Dintensions.—The vertical diameter of the face is the greatest, and extends in front from the nasal eminences of the frontal bone to the lower border of the symphysis menti; this diameter decreases as we trace it backwards. The transverse dia meter is next in length if measured at the level of the malar bone, where it is most con siderable; below and above this it gradually diminishes. The antero-posterior diameter is greatest at the level of the cheek-bones, where it extends from the cuneiform process of the occi pital bone to the anterior nasal spine of the upper maxilla ; this diameter also diminishes both above and below, but more especially below, where it comprises merely the thickness of the mental portion of the lower jaw.
The bones which form the upper jaw are united with those of the cranium above by a very irregular surface; below they are on a level with the occipital foramen, and hence that part of the face which descends below the cranium is formed exclusively by the lower jaw.
The area of the face, as presented by a vertical longitudinal section of the skull, is of a triangular figure, and forms (the lower jaw excepted) in the European about one fifth of the whole area of the skull ; in the Negro the area of the face increases in propor tion, and forms two-fifths of the whole.