Frontal Bone

The frontal bone resembles a cockle-shell in form, and consists of two portions- a vertical part, corresponding with the region of the forehead ; and a horizontal orbital plate, which enters into the formation of the roofs of the orbital and nasal cavities.

The frontal bone has two surfaces, an external and an internal.

The external surface (fig. 362) is convex and divided into a large frontal and two small temporal surfaces by lines which arch backwards across the lateral parts of the bone.

The frontal surface usually exhibits, in the lower part of the median plane, the remains of the frontal or metopic suture; in infancy this suture divides the bone into two, a condition which persists in about 9 per cent of skulls. On each side of the median plane, about 3 cm. above the supra-orbital margin, there is a rounded elevation, termed the frontal eminence. These eminences vary in size in different individuals, are occasionally asymmetric, and are especially prominent in young skulls. Below the frontal eminences, and separated from them by a shallow groove, there are two curved superciliary arches, the medial parts of which are prominent and joined to each other by a, smooth elevation named the glabella. These arches are larger in the male than in the female, and their degree of prominence depends to some extent on the size of the frontal sinuses ; prominent superciliary arches are, however, occasionally associated with small sinuses. Beneath the superciliary arches the curved supra-orbital margins form the upper portions of the orbital openings. The lateral two-thirds of each supra-orbital margin are sharp ; the medial one-third is rounded. At the junction of these two parts the supra-orbital notch, or foramen, is situated; it trans mits the supra-orbital vessels and nerve. Medial to this notch, and present in about 50 per cent, of skulls, is the small frontal notch or foramen. The supraorbital margin ends laterally in the zygomatic process, which is strong and prominent, and articulates with the zygomatic bone. From this process a line curves upwards and backwards and soon divides into the superior and inferior temporal lines.

The portion of the bone which projects downwards between the supra-orbital margins is named the nasal part. It presents an uneven interval, sometimes termed the nasal notch, which articulates on each side of the median plane with the nasal bone, and lateral to this with the frontal process of the maxilla and with the lacrimal bone. From the centre of the notch the nasal part projects downwards and forwards behind the nasal bones (fig. 333) and frontal processes of the maxilla;, and supports the bridge of the nose. The nasal part ends below in a sharp nasal (frontal) spine, and on each side of this there is a small grooved surface which forms apart of the roof of the corresponding nasal cavity. The nasal spine forms a part of the septum of the nose; in front it articulates with the crest of the nasal bones, behind with the perpendicular plate of the ethmoid bone (fig. 331).

The temporal surface, below and behind the temporal lines, forms the anterior part of the temporal fossa and gives origin to a part of the temporal muscle.

The cerebral or internal surface (fig. 363) of the frontal bone is concave. In the upper part of the median plane it is marked by a vertical groove, termed the sagittal sulcus, the edges of which unite below to form the frontal crest; the sulcus lodges the anterior part of the superior sagittal sinus, while to its margins and to the frontal crest the anterior part of the falx cerebri is attached. The crest ends below in a small notch, which is converted into the foramen caecum by articulation with the ethmoid bone. The foramen caecum varies in size in different skulls, and is rarely pervious ; when this is so, it transmits a vein from the nose to the superior sagittal sinus. On each side of the median plane the surface is marked by impressions for the cerebral gyri, and minute furrows for meningeal vessels. Several small, irregular fossae may be seen on each side of the sagittal sulcus, for the reception of axachnoid granulations. They are termed granular pits.

Figure 362
The parietal margin (border of the squama) is thick, strongly serrated, bevelled at the expense of the cerebral surface above, where it rests upon the parietal bones, and at the expense of the temporal surface on each side, where it receives the lateral pressure of the parietal bones ; it is continued below into a triangular, rough surface, for articulation with the greater wing of the sphenoid bone.

The orbital plates of the frontal bone consist of two thin triangular lamellae, which form the vaults of the orbits, and are separated from each other by a wide gap named the ethmoidal notch.

The orbital surface (fig. 363) of each orbital plate is smooth and concave, and presents, in its anterolateral part, a shallow depression which lodges the lacrimal gland and consequently is termed the fossa for the lacrimal gland, below and behind the medial end of the supra, orbital margin, about midway between the supra-orbital notch and the frontolacrimal suture, there is a small depression or spine [the fossa vel spina trochlearis], for the attachment of the fibrocartilaginous pulley of the superior oblique muscle. The cerebral surface is convex, and marked by impressions for the gyri on the inferior surface of the frontal lobe of the brain, and by faint grooves for the meningeal branches of the ethmoidal vessels.

Figure 363
Figure 364
The ethmoidal notch (fig. 363) separates the two orbital plates; it is quadrilateral and filled, in the articulated skull, by the cribriform plate of the ethmoid bone. On the margins of the notch are portions of several air-sinuses which complete the ethmoidal sinuses when the ethmoid bone is in position. Two transverse grooves cross each margin of the notch ; they are converted into the anterior and posterior ethmoidal canals by the ethmoid bone, and open on the medial wall of the orbit; they transmit the anterior and posterior ethmoidal nerves and vessels.

Figure 365
The openings of the frontal sinuses (fig. 363) are situated in front of the ethmoidal notch, and lateral to the nasal spine. These sinuses are two irregular cavities. which extend backwards, upwards, and laterally for a variable distance between the tables of the frontal bone; they are separated from each other by a thin bony septum, which is often deflected to one or other side of the median plane, with the result that the sinuses are seldom symmetrical. Rudimentary at birth, the frontal sinuses are usually fairly well-developed between the seventh and eighth years, but reach their full size only after puberty. They vary in size in different persons and are larger in men than in women. ^* Occasionally they extend backwards in the roofs of the orbital cavities as far as the optic foramina. Each communicates with the middle meatus of the corresponding nasal cavity by means of a passage called the frontonasal duct.

The posterior borders of the orbital plates are thin and serrated, and articulate with the lesser wings of the sphenoid; the lateral part of each usually appears in the middle fossa of the skull between the greater and lesser wings of the sphenoid bone.

Structure.-The frontal bone is thick and consists of spongy substance contained between two compact lamina ; the spongy substance is absent in the regions occupied by the frontal sinuses. The orbital plate, composed entirely of compact bone, is thin and translucent in its posterior two-thirds.

Ossification (fig. 365).-The frontal bone is ossified in membrane from two primary centres which appear in the seventh or eighth week of fetal life, one above each supraorbital margin From each of these centres ossification extends upwards to form the corresponding half of the bone, and backwards to form the orbital plate. The nasal spine is ossified from two secondary centres, one on each side of the median plane ; secondary centres also appear in the nasal parts and zygomatic processes. At birth the bone consists of two pieces separated by the frontal suture, but union of the pieces begins in the second year, and the frontal suture is usually obliterated, except at its lower part, by the eighth year.

 

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