Mandible

The mandible consists of a curved, horizontal portion, the body, and two perpendicular portions, the rami, which unite with the ends of the body nearly at right angles (angle of the jaw). It articulates with both temporal bones at the mandibular fossa at the temporomandibular joints (TMJ).

Body

The body (aka corpus mandibulæ or horizontal portion) is curved somewhat like a horseshoe and has two surfaces (external and internal) and two borders (superior and inferior). 

The external surface is marked in the midline line by a faint ridge, indicating the symphysis. This ridge divides below and encloses the mental protuberance, a triangular eminence the base of which is depressed in the center but raised on either side to form the mental tubercle. On either side of the symphysis, just below the incisor teeth, is a depression, the incisive fossa, which gives origin to the mentalis muslce and a small portion of the orbicularis oris muscle. Below the second premolar tooth, on either side, midway between the upper and lower borders of the body, is the mental foramen, for the passage of the mental vessels and and mental nerve (branch of the inferior alveolar nerve - CN Vc).

The internal surface is concave from side to side. Near the lower part of the symphysis is a pair of laterally placed spines, termed the mental spines, which give origin to the genioglossi muscles. Immediately below these is a second pair of spines, or more frequently a median ridge or impression, for the origin of the geniohyoid muscles. In some cases the mental spines are fused to form a single eminence, in others they are absent and their position is indicated merely by an irregularity of the surface. Above the mental spines a median foramen and furrow are sometimes seen; they mark the line of union of the halves of the bone. Below the mental spines, on either side of the middle line, is an oval depression for the attachment of the anterior belly of the digastric muscle. Extending upward and backward on either side from the lower part of the symphysis is the mylohyoid line, which gives origin to the mylohyoid muscle; the posterior part of this line, near the alveolar margin, gives attachment to a small part of the superior constrictor, and to the pterygomandibular raphé. Above the anterior part of this line is a smooth triangular area against which the sublingual gland rests, and below the posterior part, an oval fossa for the submandibular gland.

The superior or alveolar border, wider behind than in front, is hollowed into cavities, for the reception of the teeth - sixteen in total. To the outer lip of the superior border, on either side, the buccinator muscle is attached as far forward as the first molar tooth.

The inferior border is rounded, longer than the superior, and thicker in front than behind; at the point where it joins the lower border of the ramus a shallow groove; for the facial artery, may be present.

Ramus

The Ramus (aka ramus mandibulæ or perpendicular portion) is quadrilateral in shape, and has two surfaces (external and internal), four borders (anterior, superior, posterior, inferior), and two processes (coronoid porcuess and condylar process).

The external (lateral) surface is flat and marked by oblique ridges at its lower part and it gives attachment throughout nearly the whole of its extent to the masseter muscle.

The inner (medial) surface has the mandibular foramen, into which passes the inferior alveolar vessels and nerve, in its cernter. The margin of this opening is irregular; it presents in front a prominent ridge, surmounted by a sharp spine, the lingula mandibulæ, which gives attachment to the sphenomandibular ligament; at its lower and back part is a notch from which the mylohyoid groove runs obliquely downward and forward, and lodges the mylohyoid vessels and nerve. Behind this groove is a rough surface, for the insertion of the medial pterygoideus muscle.

The mandibular canal runs obliquely downward and forward in the ramus, and then horizontally forward in the body, where it is placed under the alveoli and communicates with them by small openings. On arriving at the incisor teeth, it turns back to communicate with the mental foramen, giving off two small canals which run to the cavities containing the incisor teeth. In the posterior two-thirds of the bone the canal is situated nearer the internal surface of the mandible; and in the anterior third, nearer its external surface. It contains the inferior alveolar vessels and nerve, from which branches are distributed to the teeth.

The lower border of the ramus is thick, straight, and continuous with the inferior border of the body of the bone. At its junction with the posterior border is the angle of the mandible, which may be either inverted or everted and is marked by rough, oblique ridges on each side, for the attachment of the masseter muscle laterally, and the medial pterygoideus muscle medially; the stylomandibular ligament is attached to the angle between these muscles. The anterior border is thin above, thicker below, and continuous with the oblique line. The posterior border is thick, smooth, rounded, and covered by the parotid gland. The upper border is thin, and is surmounted by two processes, the coronoid in front and the condyloid behind, separated by a deep concavity, the mandibular notch.

Coronoid Process

The coronoid(aka processus coronoideus) is a thin, triangular eminence, which is flattened from side to side and varies in shape and size. Its anterior border is convex and is continuous below with the anterior border of the ramus; its posterior border is concave and forms the anterior boundary of the mandibular notch. Its lateral surface is smooth, and affords insertion to the temporalis muscle and masseter muscle. Its medial surface also gives insertion to the temporalis muscle, and presents a ridge which begins near the apex of the process and runs downward and forward to the inner side of the last molar tooth. Between this ridge and the anterior border is a grooved triangular area, the upper part of which gives attachment to the temporalis muscle, the lower part to some fibers of the buccinator muscle.    

Condylar Process

Condylar Process (processus condyloideus or condyloid process) is thicker than the coronoid, and consists of two portions: the condyle, and the constricted portion which supports it, the neck.

The condyle presents an articular surface for articulation with the articular disk of the temporomandibular joint (TMJ); it is convex from before backward and from side to side, and extends farther on the posterior than on the anterior surface. Its long axis is directed medialward and slightly backward, and if prolonged to the middle line will meet that of the opposite condyle near the anterior margin of the foramen magnum. At the lateral extremity of the condyle is a small tubercle for the attachment of the temporomandibular ligament.

The condylar neck is flattened from front to back, and strengthened by ridges which descend from the forepart and sides of the condyle. Its posterior surface is convex; its anterior presents a depression for the attachment of the lateral pterygoideus muscle.

The mandibular notch, separating the two processes, is a deep semilunar depression, and is crossed by the masseteric vessels and nerve.