This ridge divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle.
Running backward and upward from each mental tubercle is a faint ridge, the oblique line, which is continuous with the anterior border of the ramus; it affords attachment to the depressor labii Inferioris (Quadratus labii inferioris) and depressor anguli oris (Triangularis); the platysma is attached below it.
Immediately below these is a second pair of spines, or more frequently a median ridge or impression, for the origin of the geniohyoid.
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.
Extending upward and backward on either side from the lower part of the symphysis is the mylohyoid line, which gives origin to the mylohyoid; the posterior part of this line, near the alveolar margin, gives attachment to a small part of the Constrictor pharyngis superior, and to the pterygomandibular raphé.
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.