The frontal bone is a single, unpaired, bowl-shaped bone that forms the forehead in the human skull. It is a major component of the cranium, the superior part of the skull that encases the brain. This bone shapes the upper face and provides mechanical defense for the delicate nervous tissue of the frontal lobe. Its primary role is protecting the anterior portion of the brain from external forces.
Classification as a Flat Bone
The frontal bone is categorized as a flat bone, a classification shared with most bones forming the top and sides of the skull. Flat bones are thin, broad, and curved, resembling a protective plate ideally suited to shield underlying soft tissues, particularly the brain.
The flattened shape allows the bone to distribute mechanical stress across a larger surface area, minimizing impact force on any single point. This design contrasts with long bones, which prioritize movement, or short bones, which provide stability.
Anatomical Position and Key Landmarks
The frontal bone occupies the anterosuperior aspect of the cranium, forming the forehead and the roof over the eyes. It articulates with the two parietal bones posteriorly at the coronal suture and connects with several facial bones inferiorly. The bone is divided into two main parts: the vertical squamous part that forms the forehead, and the horizontal orbital part that forms the roof of the eye sockets.
A prominent feature on the external surface is the supraorbital margin, which forms the upper rim of the orbit, or brow ridge. Just above this margin are the superciliary arches, bony elevations that meet in the midline at the glabella. The supraorbital margins often contain small openings, called the supraorbital foramina or notches, which allow the passage of nerves and blood vessels to the forehead.
The bone also houses the frontal sinuses, air-filled cavities located just behind the brow ridges and separated by a thin bony septum. These sinuses connect to the nasal cavity, contributing to the lightness of the skull and the resonance of the voice. The orbital part of the frontal bone extends inward to form a significant portion of the anterior cranial fossa, the platform upon which the frontal lobes of the brain rest.
Internal Structure
The internal structure of the frontal bone, like other cranial flat bones, consists of three distinct layers. The outer layer is a dense, compact bone called the outer table. This layer is relatively thick and tough, providing the initial defense against external blunt force trauma.
Beneath the outer table lies a layer of spongy bone known as the diploë, a subclass of cancellous bone. This middle layer is composed of a network of interconnected bony struts, or trabeculae, filled with bone marrow. The diploë functions as a shock absorber, helping to dissipate impact energy and prevent fractures from propagating through the bone’s thickness.
The innermost layer is the inner table, also made of compact bone, but it is thinner and more brittle than the outer table. This inner surface is smooth and concave, conforming to the shape of the brain beneath it. The three-layered design, with compact outer and inner tables separated by the shock-absorbing diploë, is a mechanism for brain protection.