The Bones in the Neurocranium
The skull, or more officially, the cranium, has bones that protect your brain. The bones that protect the brain are strong and have very little movement at the joints. Their main purpose is to protect the delicate tissues of the brain and its coverings.
The part of the cranium that holds your brain is called the neurocranium, which contains the cranial cavity. It has a rounded dome shape on the top and cranial base at the bottom. The neurocranium is formed by the following bones.
Frontal bone: The frontal bone is best seen from the front of the skull. It forms the forehead and top portions of the orbits (what most people call the eye sockets). It has two supercilliary arches above the eyes and two supraorbital foramina (notches) on the medial part of the arches. The frontal bone also has two hollow spaces called the frontal sinuses that are lined with mucous membranes.
Parietal bones: A pair of parietal bones form much of the sides and top of the cranium. They meet in the midline at the sagittal suture, which is a jagged immobile joint. They join the frontal bone at the coronal suture. The meeting point of the sagittal and coronal sutures is the bregma. Each parietal bone has two arched lines called the inferior and superior temporal lines. The area inferior to the inferior temporal line is part of the temporal fossa, which is a shallow depression on the side of the skull.
Occipital bone: The occipital bone forms the posterior portion of the skull. It features a large bump, the external occipital protuberance, that you can palpate in the midline just above the neck. Superior and inferior nuchal lines extend laterally from the protuberance. The occipital bone joins the parietal bones at the lambdoid suture. The point where the three bones meet is called the lambda, and sometimes it can be felt as a slight depression at the top and back of your head. The occipital bone has a large opening called the foramen magnum, which transmits the spinal cord. It also has two occipital condyles, which are two rounded bony prominences that articulate with the first cervical vertebra.
Temporal bones: Two temporal bones form the lower parts of the sides of the cranial vault (the space occupied by the brain). Each one meets a parietal bone at the squamous suture and forms part of the temporal fossa. Two processes (bony projections) extend from the inferior portion of each bone: the styloid process and mastoid process. Another process, called the zygomatic process, joins the zygomatic bone. The temporal bones also join the sphenoid bone.
Sphenoid bone: The sphenoid bone is a butterfly-shaped bone situated anterior to the temporal bones. The body has hollow spaces called the sphenoid air sinuses. It also has two larger and two smaller wings and two processes called the pterygoid plates that project inferiorly. Three pairs of openings, the foramen rotundum, foramen ovale, and foramen spinosum, are situated on the greater wings near the body. A depression called the sella turcica houses the pituitary gland. It’s bound by the two middle clinoid processes anteriorly and a portion of bone called the dorsum sella posteriorly. It also forms the posterior walls of the orbits.
The pterion is the meeting point of the sphenoid, frontal, parietal, and temporal bones. You can find it in the anterior part of the temporal fossa, superior to the middle of the zygomatic arch. Immediately deep to the pterion, inside the cranial cavity, is the middle meningeal artery. Trauma to this region may result in damage to the artery and an epidural hematoma.
Ethmoid bone: The ethmoid bone is spongy and cube shaped. It sits between the orbits at the top of the nasal cavity and has two bony plates called the superior and middle concha. The crista galli is a ridge that projects upward. The perpendicular plate of the ethmoid bone projects inferiorly into the nasal cavity to form the nasal septum.