Anatomy: The Cranium
A. Introduction
The cranium (from the Greek word krania, meaning skull) is the most cephalad aspect of the skeleton. The skull is the bony framework of the head. It composed of 22 bones (not counting the bones of the middle ears) and rest on the superior end of the vertebral column. The bones of the skull are divided into two regions: the neurocranium (which protects the brain) and the viscerocranium (which forms the face).
The skull also supports tendinous muscle attachments and allows neurovascular passage between intracranial and extracranial anatomy. The skull is embryologically derived from mesoderm and neural crest and will fuse, harden, and mold from gestation through adulthood. It gives the human face its form, and even minor variations in anatomy among individuals can lead to vast differences in appearance.
Various foramina, condyles, and other bony landmarks provide passageways and attachments for the important structures associated with the skull. Due to its complex development and associated important structures, understanding skull anatomy holds great clinical and surgical significance.
B. Structure and Function
1. Neurocranium (Cranial Bones)
The cranial bones form the cranial cavity, which encloses and protects the brain. The eight cranial bones are: (1) the frontal bone, (2) two parietal bones, (3) two temporal bones, (4) the occipital bone, (5) the sphenoid bone, and (6) the ethmoid bone.
1.1 Calvaria and Skull Base
The calvaria, the uppermost part of the skull, protects the cerebral cortex, cerebellum, and orbital contents. It is composed of the frontal bone, parietal bones, temporal bones, and occipital bone.
- The coronal suture is the transverse mid-anterior junction of the frontal bone and the two parietal bones.
- The parietal bones articulate with the temporal bones inferiorly via the squamosal sutures and the occipital bone posteriorly via the lambdoid suture.
- The sagittal suture lies along an anterior-posterior axis and is the articulation of the two parietal bones.
- The pterion is the articulation of the frontal, parietal, temporal, and sphenoid bones just superior to the pinna.
- The asterion is the articulation of the parietal, temporal, and occipital bones.
Anteriorly, the frontal bone forms the superior aspect of the orbits.
- The glabella is a key midline landmark of the frontal bone. It lies superior to the nasion and between the superciliary ridges. The frontal sinuses lie deep to the brow ridges.
- The bregma is the junction of the coronal and sagittal sutures, and lambda is the junction of the lambdoid and sagittal sutures.
- The temporal bones subdivide into petrous, squamous, zygomatic, and mastoid parts.
- The petrous portion houses the inner ear.
- The mastoid is a bony prominence that lies posterior to the auricle and has an associated sinus.
- The occipital bone is the most posterior aspect of the skull.
1.2 Intracranial Fossae
There are three cranial fossae with various structural landmarks.
- The anterior cranial fossa forms from the frontal bone, the sphenoid bone, and the ethmoid bone.
- The middle cranial fossa forms from the sphenoid bone and two temporal bones.
- Finally, the posterior cranial fossa forms from the occipital bone and two temporal bones.
- Anterior Cranial Fossa (contains frontal lobe of the brain)
- Cribriform plate
- Middle Cranial Fossa (contains temporal lobe of the brain)
- Optic canal
- Superior orbital fissure
- Foramen spinosum
- Foramen rotundum
- Foramen ovale
- Posterior Cranial Fossa (contains the cerebellum)
- Internal auditory meatus
- Jugular foramen
- Foramen magnum
- Hypoglossal canal
Skull Foramen. Skull foramen illustration includes cribriform plate of ethmoid bone, optic canal, foramen rotundum, foramen lacerum, foramen ovale, foramen spinosum, carotid canal, internal acoustic meatus, jugular foramer, hypoglossal canal, and foramen magnum. Contributed by Beckie Palmer.
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