The cranium, often recognized as the braincase or neurocranium, forms the upper and posterior part of the skull. Its primary role involves enclosing and protecting the brain, along with its surrounding membranes and blood vessels. This bony structure acts as a protective shield for the central nervous system. The cranium is composed of several bones that are fused together, forming a robust enclosure.
Identifying the Cranial Bones
The human cranium consists of eight distinct bones that work together to create its protective structure. The frontal bone makes up the forehead and extends to form the upper parts of the eye sockets. Behind the frontal bone, two parietal bones are situated, forming a significant portion of the top and sides of the head. Below each parietal bone, on the sides of the skull, are the two temporal bones.
The occipital bone is located at the very back of the skull, providing its posterior boundary. Deeper within the skull, the sphenoid bone is found below the frontal bone, spanning across the width of the skull and forming a large part of its base. Lastly, the ethmoid bone is an intricate bone positioned in front of the sphenoid bone, contributing to the nasal cavity and the medial wall of the eye sockets.
Unpacking Their Names: The Logic Behind Each Bone
The names of the cranial bones often reflect their location, shape, or historical understanding.
Frontal Bone
The frontal bone derives its name directly from the Latin word “frons,” which means “forehead.” This naming is straightforward, as the bone prominently forms the forehead and the anterior aspect of the cranium. Its position at the very front of the skull makes this designation fitting.
Parietal Bones
The two parietal bones get their name from the Latin word “paries,” meaning “wall.” These bones form the side walls and much of the roof of the cranium. They meet at the top of the head along a major suture.
Occipital Bone
The occipital bone is named for its position at the back of the head. The term “occipital” relates to the “occiput,” the posterior part of the head. This bone forms the lower and back part of the cranium and contains a large opening, the foramen magnum, through which the spinal cord connects to the brain.
Temporal Bones
The two temporal bones are located on the sides of the skull, near the temples. The etymology of “temporal” comes from the Latin word “tempus,” meaning “time.” One theory suggests this name relates to the area of the head where gray hairs often first appear as a sign of aging. Another possible connection is to the pulsations of the underlying superficial temporal artery.
Sphenoid Bone
The sphenoid bone has a name reflecting its unique shape and position. Its name originates from the Greek word “sphenoeides,” which translates to “wedge-shaped.” This bone is centrally located at the base of the skull, often described as resembling a butterfly or a bat with outstretched wings, and it appears to be wedged between other cranial bones. It articulates with many other cranial bones.
Ethmoid Bone
Finally, the ethmoid bone is named from the Greek word “ethmos,” meaning “sieve.” This name describes its structure, as the ethmoid bone contains numerous small perforations. These sieve-like openings, particularly in its cribriform plate, allow the olfactory nerves to pass from the nasal cavity to the brain.