The Frontal Bone

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The frontal bone is a bone of the skull found in the forehead region. It is one of eight bones that form the cranium, or brain case. The frontal bone plays a vital role in supporting and protecting the delicate nervous tissue of the brain. It gives shape to the skull and supports several muscles of the head.

Anatomy

The frontal bone is a bowl-shaped bone in the frontal (forehead) region of the skull. It is located superior to the nasal bones and maxillae and anterior to the parietal bones. At its inferior border, the frontal bone forms the roof of the orbits and the brow. The coronal suture forms the posterior boundary of the frontal bone where it meets the parietal bones.

Several important landmarks are found on the frontal bone. Just above the orbits is a thickened region of bone known as the supraorbital ridge. This region corresponds to the brow of the skull and forms the anterior cover for the frontal sinuses. Two small holes, known as the supraorbital foramina, are found above each orbit in the supraorbital ridge and provide exit points to the forehead for the supraorbital nerve and the supraorbital artery.

At the inferior end of the supraorbital ridge are the supraorbital margins, where the frontal bone makes a sharp angle to form the superior and medial margins of the orbits. Inside the orbits, the frontal bone continues inferiorly along the medial margin until it meets the ethmoid and lacrimal bones. It continues posteriorly along the superior margin until joining with the sphenoid bone.

Medial to the supraorbital ridges is the glabella, a small depression that ends inferiorly at the nasal bones. Superior to the glabella and supraorbital ridges is the squamous region, where the frontal bone is smooth and slopes posteriorly toward the partietal bones. A midsagittal suture is often present in the glabella and squamous region, especially in infants and young children. This frontal suture is indicative of the frontal bone's prenatal development from two individual fetal bones.

The interior surface of the frontal bone contains many shallow depressions and slight ridges that follow the contours of the frontal lobe of the brain. A long, straight, midsagittal ridge known as the frontal crest extends from the frontal bone to follow the longitudinal fissure of the brain until it merges with the crista galli of the ethmoid bone at its inferior end.

Deep to the supraorbital ridges is a pair of hollow spaces known as the frontal sinuses. The frontal sinuses connect to the nasal cavity and, like the nasal cavity, are lined with mucous membrane. The exact function of the frontal sinuses is not certain, but it is believed that they reduce the weight of the skull by being hollow and increase the resonance of the skull to improve vocal tone.

Physiology

The primary functions of the frontal bone are the protection of the brain and the support of the structures of the head. The hard mineral matrix of the frontal bone provides protection for the soft brain tissue. Although the frontal bone follows the ridges of the brain very closely, a small gap between the frontal bone and brain houses the meninges and the cerebrospinal fluid of the cranium. The pressure exerted by cerebrospinal fluid on the interior of the cranium holds the brain in place and prevents the brain from colliding with the skull.

Two major muscles of the face --- the temporalis and orbicularis oculi --- form origins on the frontal bone. The frontalis muscle, which forms the frontal belly of the epicranius muscle, passes over the smooth squamous region of the frontal bone to attach the skin of the brow to the galea aponeurotica on top of the head.

The frontal bone is classified as a flat bone due to its relatively thin and flat shape. Like all flat bones, the frontal bone has spongy bone in its center, surrounded by a thin layer of compact bone on its inner and outer surfaces. Between the columns of spongy bone is red bone marrow that produces blood cells through hematopoiesis. Pluripotent stem cells in the red marrow divide repeatedly throughout a lifetime to produce many daughter cells. These daughter cells differentiate to form all of the red blood cells, white blood cells, and platelets that circulate in the blood.