The internal surface of the body of the lower jaw is concave, and presents in the median line, at the symphysis, a vertical crest (erista mentalis interna), which is not so distinct as the corresponding ridge on the outer surface of the bone : at its lower extremity is a tubercle having four summits (the genial processes, viyft0Y, chin, spina interna,) which give attachment to two pairs of muscles, viz. the two superior genial processes to the genio hyo-glossi, the two inferior to the genio hyoidei : below and to the outer side of these processes, on the lower border of the bone, are two oval rough depressions, one on each side of the symphysis, for the attachment of the anterior bellies of the digastric muscles. From the genial processes proceeds obliquely upwards and backwards, to join the anterior border of the ramus of the jaw, the internal oblique line, or the mylo-hyoid ridge. It is distinctly marked and very prominent oppo site the last molar tooth ; like the external oblique line it divides the bone diagonally into two triangular portions, the anterior of which, situated above and in front of the ridge, is smooth, concave, and to the outer side of the genial processes presents' a depression (sublingualAssa ) for the reception of the sub lingual gland : elsewhere this surface is lined by the gums, and forms the inner wall of the alveolar cavities; but it is destitute of the ridges and depressions which are seen on the outer surface of the bone. The triangular surface below the oblique line is marked by numerous small holes for the passage of nu tritious vessels, and by a large depression (the submaxillary fossa) for the reception of the submaxillary gland. The two oblique maxillary lines which have been just described divide the body of the jaw into two portions, one superior or alveolar, the other inferior or basilar : in the foetus the former predominates considerably ; in the adult they are nearly equal, and in the edentulous jaw of old age the body almost entirely consists of the basilar portion.
The upper or alveolar border forms a lesser curve than that of the alveolar border of the superior maxilla : like it, however, it presents sockets for the reception of sixteen teeth, which vary also in form and depth in correspondence with the fangs of the teeth which they lodge. The orifices of the sockets, however, take a direction different from those of the upper jaw, for while the sockets of the upper incisors look downwards and forwards, those of the lower are directed upwards and backwards; and again the alveoli of the upper canine and molar teeth look downwards and outwards, whereas those of the lower are directed up wards and inwards: hence, from this different inclination of the teeth in the two jaws, and from the larger curve described by the alveolar border of the superior maxilla, we find that when the mouth is closed the upper front teeth cover the lower and at the sides overhang them a little. This arrangement is favourable to the division and mastication of the food.
The lower border or base is smooth and thick, and forms a larger curve than the upper, so that the surfaces of this jaw have an in clination from above downwards and forwards: it forms the oval border of the lower part of the face, and is the strongest portion of the bone.
The rami are flat, quadrilateral processes, which stand up from the body of the jaw at almost a right angle : in the child and old person this angle is much more obtuse. Each ramus presents two surfaces and four borders.
The external or masseteric surface has an inclination from above downwards and more or less outwards : it is rough, especially below, where it presents some irregular oblique ridges and depressions for the attachment of the masseter: in front of these marks, near the lower border of the bone, there is often a slight groove, which indicates the course of the facial vessels.
The internal or pterygoid surface is also rough below for the attachment of the internal pterygoid muscle. In its centre is the spreading superior orifice (superior dental foramen) of the lower dental canal, marked and partly hidden internally by a spine, which gives attachment to the internal lateral ligament of the temporo-maxillary articulation : from this hole, taking a direction downwards and for wards is a groove (the mylo-hyoid groove), which lodges the branch of the inferior dental artery and nerve.
The borders of the rami are, an anterior or buccal, grooved below, where it corre sponds with the alveolar border of the bone; the margins of this groove, which are con tinuous with the oblique lines of the bone, unite above and form a sharp convex edge. The posterior or parotid border is round and thick above, and narrow below, and is em braced by the parotid gland : inferiorly and internally it gives attachment to the stylo maxillary ligament. The superior or zygomatic border is sharp and concave, forming a notch (the signioid notch ), which looks upwards. The inferior border is rounded, and is con tinuous with the lower border of the body.
The angles of the lower jaw are formed by the union of the body and rami ; each is turned a little outwards, and in the adult forms nearly a right angle ; in the infant and in the old person it is obtuse. This part of the bone is prominent and separates the in sertion of the masseter and internal pterygoid muscles.
On the upper part of each ramus stand two processes, which are separated by the sigmoid notch ; the anterior is the coronoid, which is of a triangular form, flattened laterally, and sharp in front and behind ; its summit is somewhat rounded : this process gives at tachment to the temporal muscles. The con dyloid process is situated behind the sigmoid notch, and arises from the ramus by a narrow neck, which is directed upwards and a little inwards, swelling above Into an oval head or condyle, that has an articular surface on its summit. This articular surface is trans versely oval, convex, covered in the recent subject with cartilage, and inclines from within outwards and a little forwards. The condyle, from the direction of its neck, somewhat over hangs the internal surface of the ramus ; it is articulated with the anterior division of the glenoid cavity of the temporal bone. The direction and form of its articular surfaces are calculated to facilitate the rotatory move ments of the lower jaw during mastication. In front the neck of the condyle presents a depression for the attachment of the external pterygoid muscle.