Squamous part of the frontal bone
|Squamous part of the frontal bone|
Frontal bone. Outer surface. (The squamous part is the upper two thirds.)
Frontal bone. Inner surface. (The squamous part is the upper two thirds.)
|Anatomical terms of bone|
There are two surfaces of the squamous part of the frontal bone: the external surface, and the internal surface.
The external surface is convex and usually exhibits, in the lower part of the middle line, the remains of the frontal suture; in infancy this suture divides the frontal bone into two and later fuses. A condition where fusion has not taken place, may persist throughout life and is referred to as a metopic suture.
On either side of this suture, about 3 cm. above the supraorbital margin, is a rounded elevation, the frontal eminence (tuber frontale).
These eminences vary in size in different individuals, are occasionally unsymmetrical, and are especially prominent in young skulls; the surface of the bone above them is smooth, and covered by the galea aponeurotica.
Below the frontal eminences, and separated from them by a shallow groove, are two arched elevations, the superciliary arches; these are prominent medially, and are joined to one another by a smooth elevation named the glabella. They are larger in the male than in the female, and their degree of prominence depends to some extent on the size of the frontal air sinuses; prominent ridges are, however, occasionally associated with small air sinuses.
Beneath each superciliary arch is a curved and prominent margin, the supraorbital margin, which forms the upper boundary of the base of the orbit, and separates the squamous part from the orbital portion of the bone.
The lateral part of this margin is sharp and prominent, affording to the eye, in that situation, considerable protection from injury; the medial part is rounded.
Running upward and backward from this process is a well-marked line, the temporal line, which divides into the upper and lower temporal lines, continuous, in the articulated skull, with the corresponding lines on the parietal bone.
Between the supraorbital margins the squamous part projects downward to a level below that of the zygomatic processes; this portion is known as the nasal part and presents a rough, uneven interval, the nasal notch, which articulates on either side of the middle line with the nasal bone, and laterally with the frontal process of the maxilla and with the lacrimal.
The term nasion is applied to the middle of the frontonasal suture. From the center of the notch the nasal process projects downward and forward beneath the nasal bones and frontal processes of the maxillæ, and supports the bridge of the nose.
The nasal process ends below in a sharp spine, and on either side of this is a small grooved surface which enters into the formation of the roof of the corresponding nasal cavity.
The internal surface of the squamous part is concave and presents in the upper part of the middle line a vertical groove, the sagittal sulcus, the edges of which unite below to form a ridge, the frontal crest; the sulcus lodges the superior sagittal sinus, while its margins and the crest afford attachment to the falx cerebri.
This foramen varies in size in different subjects, and is frequently impervious; when open, it transmits a vein from the nose to the superior sagittal sinus.
On either side of the middle line the bone presents depressions for the convolutions of the brain, and numerous small furrows for the anterior branches of the middle meningeal vessels.