The bones of the cranium articulate with each other by way of sutures, which are fibrous joints between two interlocking denticulate edges which allow very little movement. This type of joint is very different from the more familiar synovial joints, highly mobile joints encased by a sac and lubricated with fluids. Most familiar joints such as the knee, elbow, and hip are synovial joints. The only synovial joints in the skull are the jaw joint and the articulations of the tiny auditory ossicles. The sutures between bones of the cranium are most evident in young individuals. Over time the bones fuse together such that some sutures in extremely old individuals are virtually invisible.
The ectocranial aspect of the frontal squama bears superciliary arches, or brow ridges, just above the supraorbital margins. The supraorbital margin may be decorated by a supraorbital foramen or notch, which transmits vessels and nerves to the forehead. Above the superciliary arches are the frontal emminences, which are swollen portions of the squama. On the lateral side of the squama, the temporal lines are visible. These lines mark the attachment site of the temporalis, a powerful elevator of the jaw. Like many muscle attachment sites, these lines are best developed in more heavily-muscled individuals.
From an inferior view, the horizontal portion has a gap, the ethmoid notch, in which the ethmoid bone rests. Just anterior to this notch are the frontal sinuses, which are air gaps lined with mucous membranes in life. The most lateral projections of the horizontal portion are the zygomatic processes, which articulate with the zygomatic bone.
In posterior view, the endocranial surface of the frontal is visible. This surface bears meningial grooves, traces of the meningial arteries. The midline of the endocranial surface has a large crest called the frontal crest. This serves as the attachment site for the falx cerebri, a tough fibrous sheet which separates the two hemispheres of the brain.
Intact frontals are easily identified. When fragmentary, the horizontal portion can be identified because the frontal is the only bone which forms the top of the eye orbit. Furthermore, only the sphenoid and maxilla have sinuses comparable in size to the frontal sinus. The squamous portion may be mistaken for a parietal, but the latter have much deeper meningeal grooves.
The parietals are rectangular in shape (almost square). Most of the ectocranial surface is basically smooth, marked only by the continuation of the temporal lines from the frontal. Near the middle of the ectocranial surface is a rounded protrusion which marks the center of ossification of the bone. This protrusion is the parietal eminence. The inferior margin of the parietal is quite distinct from the other borders. It is beveled and striated in the region where it is overlapped by the temporal bone at the squamous suture. The remaining borders (at the coronal, sagittal, and lamdoidal sutures) are denticulate.
The endocranial surface of the parietals is dominated by meningeal grooves which run posteriorly and superiorly. These are similar in appearance to the grooves on the frontal, but are deeper and more pronounced on the parietals.
Siding the Parietals
The meningeal grooves are the key to siding parietals. The grooves are on the endocranial surface and sweep both superiorly and posteriorly. The beveled edge of the parietal is inferior. The anterior border (at the coronal suture) is straighter than the more curved posterior border (at the lamdoidal suture).
The ectocranial surface of the squamous portion of the occipital bears the superior nuchal line and inferior nuchal line marking the attachment sites for the nuchal muscles which support and rotate the head. Between these lines, the external occipital protuberance projects from the midline of the squamous portion. The midline of the endocranial surface is marked by an internal occipital protuberance and an occipital crest inferior to the protuberance.
The condylar portions bear the large occipital condyles, which articulate with the atlas. Just posterior to the condyles are the condyloid foramina which transmit veins. The hypoglossal canals transmit the hypoglossal nerve through the anterior portion of each condyle. Each condylar portion bears a laterally projecting jugular process.
The ectocranial surface of the basilar portion of the occipital is relatively smooth, save for the pharyngeal tubercule. The anterior portion bears a roughened articular surface for the sphenoid.
The squamous is the flat superior portion of the temporal which contributes to the neurocranium. The ectocranial surface of the squamous is smooth and marked by few characteristic features. Near the inferior border of the squamous is the zygomatic process which projects anteriorly to meet the zygomatic. Just inferior to the zygomatic process on its posterior end is the mandibular fossa (glenoid fossa).
This fossa receives the mandibular condyle to form the jaw joint (or tempo-mandibular joint, TMJ). The mandibular fossa is bordered by the articular eminence anteriorly and the post-glenoid process posteriorly. Endocranially, the superior margin of the squamous is sharply beveled. The beveled superior margin overlaps the parietal bone at the squamous suture. Inferior to this beveled edge, the surface is marked by middle meningeal grooves.
The tympanic portion makes up the external ear region. The tympanic is ring-shaped, framing the opening of the external auditory meatus (EAM). The EAM is the external opening for the ear canal. Just inferior to the EAM is the vaginal process, a plate of bone which surrounds the inferiorly projecting styloid process.
The petrous (from the Latin meaning “rock”) is a bulky process which projects anteriorly from the endocranial surface of the temporal. Because the petrous is so robust, it is often preserved intact even in very fragmentary specimens. The petrous houses the organs of hearing and balance, including the tiny auditory ossicles. In medial view, the internal auditory meatus (IAM) is visible as a large hole in the petrous. In inferior view, the large carotid foramen on the medial-anterior portion of the petrous transmits the internal carotid artery. Posterior to this foramen is the jugular fossa, a depression which cradles the jugular vein. Lateral to this fossa is the stylomastoid foramen, just posterior to the styloid process. The stylomastoid foramen transmits the facial nerve.
The mastoid is the posterior portion of the temporal. In lateral view, the rough, pitted mastoid process is visible. This process projects anteriorly and inferiorly. The mastoid process is usually more robust in males than in females. A deep mastoid groove lies just medial to the mastoid process.
Siding the Temporals
The medial aspect of the temporal is marked by the beveled superior margin of the squamous and bears the petrous. The lateral aspect bears the mastoid process and the EAM. The mastoid process is on the posterior part of the temporal and projects anteriorly and inferiorly. The zygomatic process projects anteriorly.
The body of the sphenoid is the central portion from which the wings emanate. In superior view, the sella turcica (“Turkish saddle”) is visible as a deep depression which cradles the pituitary gland in life. The sella turcica is bordered posteriorly by the posterior clinoid processes and anteriorly by the anterior clinoid processes, the posterior projections of the lesser wings which rise from the body. In anterior view, the large sphenoidal sinuses are visible. In posterior view, the roughened articular surface for the occipital occurs posterior and inferior to the sella turcica.
The lesser wings project laterally from the superior portion of the body. The optic canals are visible in posterior view near the junction of the lesser wings and the body. These canals transmit the optic nerve from the brain to the eyeballs.
The greater wings project superiorly and laterally from the inferior portion of the body. Their endocranial surfaces (posterior-superior) form much of the middle cranial fossa. The orbital surfaces on the anterior aspect of the wings form much of the lateral wall of the orbit. The gap between the lesser wing and the orbital surface of the greater wing is termed the superior orbital fissure. This fissure can be seen in the posterior wall of the orbit in an intact cranium. The area of contact between the greater wings and the body is pierced by the foramen rotundum, which transmits the maxillary nerves and vessels. Just posterior to this foramen are the foramen ovale and foramen spinosum, which transmit mandibular nerves and vessels.
The pterygoid processes dangle inferiorly from the body of the sphenoid. Each process is divided into two plates (medial and lateral). The inferior tip of each medial plate ends in a hook-shaped pterygoid hamulus. The pterygoid plates provide the site of origin for the medial pterygoid muscles, which are important elevators of the mandible.