Norma Basalis (internal view)
CRANIAL FOSSA: The internal aspect of the cranial base is naturally divided into three regions. The Cranial Fossae are 1) The anterior cranial fossa from the internal surface of the frontal bone posteriorly to the lesser wings of the sphenoid bone. 2) The middle cranial fossa extends posteriorly from the lesser wings of the sphenoid bone and sphenoid body posteriorly to the ridge of the petrous temporal bone. 3) The posterior cranial fossa extends posteriorly from the ridge of the petrous temporal bone to the occipial bone at the back of the skull.
The frontal bone forms the anterior wall and most of the floor of the anterior cranial fossa.
The ethmoid bone completes the anterior part of the floor of the fossa by filling a narrow, median slot in the frontal bone.
The sphenoid bone serves as a keystone in the construction of the skull floor. For this reason, and because of its highly irregular shape, it deserves (requires?) special attention.
The body of the sphenoid, which occupies the central position in the middle cranial fossa, is indented from above by the hypophyseal fossa to form the sella turcica. It meets the ethmoid bone and so completes the central section of the floor of the anterior cranial fossa.
The paired lesser wings extend laterally from the upper, anterior portion of the body toward the walls of the skull. The lesser wings form the rear edge of the floor of the anterior cranial fossa.
The paired greater wings also extend laterally but from low on the sides of the body. They form the anterior walls of the middle cranial fossa and contribute largely to its floor and lateral walls.
The dorsum sellae, the high back of the sella turcica, is a vertical plate of bone. The anterior surface of the dorsum faces the middle cranial fossa, its posterior surface faces the posterior cranial fossa, and its superior margin serves as a boundary between the two.
The paired temporal bones, like the sphenoid, are irregular in shape, possess several distinct parts, and contribute to numerous important cranial features. Many of these attributes can be appreciated when looking at the internal skull base.
The squamous portion (L. squamosus = scaly) of the temporal bone is for the most part thin and flat. It forms much of the lateral wall of the middle cranial fossa and thus contributes to the wall of the skull as seen in norma lateralis. It also includes the posterior part of the zygomatic arch, and the mandibular fossa, which lies below the floor of the middle cranial fossa.
The petrous portion of the bone is shaped roughly like a pyramid projecting inward from the lateral wall of the skull. From its broad base, it tapers medially and anteriorly toward its apex, which lies against the body of the sphenoid. The anterior surface of the petrous pyramid contributes to the rear wall of the middle cranial fossa; its posterior surface contributes to the front wall of the posterior cranial fossa; and its summit forms a boundary between the two fossae. The petrous bone also contains the cochlear and vestibular labyrinths and provides the roof of the middle ear cavity.
The mastoid portion of the temporal bone projects downward from the junction of the petrous and squamous parts. Very little of the mastoid portion can be seen in the internal skull base; the mastoid process itself has been seen clearly in norma lateralis and norma basalis.
The occipital bone makes up the floor and most of the rear wall of the posterior cranial fossa. It also contributes to the anterior wall of the fossa.
The paired parietal bones contribute to formation of the lateral walls of all three cranial fossae.
The frontoethmoidal sutures are paired and parallel to one another. They run anterior-to-posterior along either side of the ethmoid, where it meets the frontal bone.
The sphenofrontal sutures run transversely across the floor of the anterior cranial fossa, between the frontal bone and the lesser wings of the sphenoid. Medially, each turns forward to join the frontoethmoidal suture. Laterally, beyond the tip of the lesser wing of the sphenoid, this suture turns posteriorly toward pterion, where it joins an intersection with the coronal and squamosoparietal sutures.
The sphenoparietal suture is the core of pterion. It runs horizontally from the intersection of the coronal and sphenofrontal sutures to that of the sphenosquamous and squamosoparietal sutures (see below?).
Sutures involving the temporal bone can be confusing if they are approached only in terms of their names. However, virtually all are visible on the internal surface of the skull and can be organized in structural terms. The temporal bone is outlined by a system of sutures involving the different parts of the temporal bone itself and all of the surrounding bones: sphenoid in front, parietal above, and occipital behind. Begin at the petrous apex.
The sphenopetrosal suture (fissure) runs posterolaterally from the foramen lacerum, between the greater wing of the sphenoid and the petrous bone. It lies just above the cartilage of the auditory tube.
The sphenosquamous suture is continuous with the sphenopetrosal suture and lies between the greater wing of the sphenoid and the squamous portion of the temporal bone. It arcs forward, upward, then back to enter pterion and reach the sphenoparietal suture.
The squamosoparietal suture begins in pterion, at the intersection of the sphenosquamous and sphenoparietal sutures. Lying between the squamous temporal and the parietal bones, it continues the arc upward and back before descending above the base of the petrous pyramid to reach the lambdoidal suture.
The occipitomastoid suture begins at the intersection of the squamosoparietal and lambdoidal sutures. Formed between the mastoid part of the temporal bone and the occipital bone, it descends and then turns forward across the floor of the posterior cranial fossa to reach the petrous bone.
The petrooccipital suture is continuous with the occipitomastoid suture. It runs medially and forward between the petrous pyramid and the occipital bone, reaches the petrous apex, and completes the encirclement of the temporal bone.
The lambdoidal suture, between the occipital and parietal bones, descends obliquely forward to end in the intersection of squamosoparietal and occipitomastoid sutures.
NEUROVASCULAR EXIT PORTALS:
The cribriform plate (L. cribrum=sieve) is that horizontal part of the ethmoid bone between the midline and the frontoethmoidal suture. It is thin, perforated with many small holes (hence its name), and easily fractured. It forms the roof of the nasal cavity on each side, just above the olfactory epithelium, and bundles of olfactory axons pass upward through the perforations to end in the olfactory bulb, which lies on the upper surface of the plate.
Each optic canal opens just medial to the anterior clinoid process. It passes anterolaterally through the base of the lesser wing of the sphenoid bone to enter the apex of the orbit, transmitting the optic nerve, ensheathed by meninges, and the ophthalmic branch of the internal carotid artery. As they enter the middle cranial fossa from the orbits, the optic nerves exchange axons returning from the nasal half of each retina, forming the optic chiasm (from chi –X- the 22nd letter of the Greek alphabet) against the body of the sphenoid bone in front of the pituitary gland.
All three divisions of the trigeminal nerve exit the cranial cavity from the middle cranial fossa.
The superior orbital fissure lies between the lesser and greater wings of the sphenoid bone. Through it pass the branches of the ophthalmic division of the trigeminal (V1), the oculomotor (III), trochlear (IV), and abducens (VI) nerves, and the superior ophthalmic vein. In a standard view of the internal skull base, the fissure is hidden by the overhang of the lesser wing of the sphenoid.
The foramen rotundum (L. rotundus=like a wheel) lies in the floor of the middle cranial fossa at the junction of the body and greater wing of the sphenoid. It transmits the maxillary division of the trigeminal nerve (V2) from the cranial cavity into the pterygopalatine fossa. Its true shape is difficult to appreciate from above, but it is clearly round when examined from the pterygopalatine fossa.
The foramen ovale (L. ovum=egg) is a large oval opening in the posterior part of the greater wing of the sphenoid, half-way across the floor of the middle cranial fossa. It transmits the mandibular division of the trigeminal nerve (V3) from the cranial cavity into the infratemporal fossa, along with the lesser petrosal branch of the glossopharyngeal nerve (IX). The accessory meningeal branch of the maxillary artery passes upward into the cranial cavity through the foramen along with its postganglionic sympathetic nerve plexus.
The foramen spinosum is a small round hole in the greater wing of the sphenoid just posterolateral to the foramen ovale. The middle meningeal branch of the maxillary artery passes upward through the foramen into the middle cranial fossa, accompanied by its postganglionic sympathetic nerve plexus and the nervus spinosus, a recurrent meningeal branch of the mandibular division of the trigeminal nerve.
The cranial entrance of the carotid canal lies at the apex of the petrous pyramid. The internal carotid artery, accompanied by its postganglionic sympathetic nerve plexus, emerges from the petrous bone onto the upper surface of the cartilage of the foramen lacerum. As it turns upward into the middle cranial fossa, it makes a distinct groove on the posterolateral aspect of the body of the sphenoid bone.
The internal auditory (acoustic) meatus opens on the posterior surface of the petrous temporal bone, just below the ridge. The vestibuloacoustic nerve (VIII) passes through the meatus on its way to sensory hair cells of the cochlea and the vestibular labyrinth. The nerve is accompanied by the labyrinthine artery from the anterior inferior cerebellar branch of the basilar artery. The facial nerve (VII) also enters the meatus, from which the facial canal begins its complicated course through the temporal bone to the stylomastoid foramen.
The jugular foramen is a large, irregular widening of the petrooccipital suture below the internal auditory meatus. The sigmoid and inferior petrosal dural venous sinuses pass through the foramen to join the internal jugular vein in the jugular fossa. The glossopharyngeal (IX), vagus (X), and spinal accessory (XI) nerves also exit the posterior cranial fossa via the jugular foramen
The hypoglossal canals pass anterolaterally through the occipital bone immediately above the occipital condyles. They transmit the hypoglossal (XII) nerves.
The foramen magnum is the large slightly oval opening in the center of the floor of the posterior cranial fossa. The brain stem becomes continuous with the spinal cord through the foramen, while the spinal accessory nerves (XI) pass upward from cervical cord segments into the cranial cavity. The vertebral arteries also ascend through the foramen on their way to form the basilar artery and so enter the circle of Willis.
Prominent landmarks & reference points:
Anterior cranial fossa:
The cribriform plate (L. cribrum=sieve) is that horizontal part of the ethmoid bone between the midline and the frontoethmoidal suture. It is thin, perforated with many small holes (hence its name), and easily fractured. of the ethmoid bone fills the median slot in the frontal bone. It forms the roof of the nasal cavities immediately above the olfactory epithelium.
The crista galli (cock’s comb) is a sail-shaped (triangular?) vertical plate of bone projecting upward along the midline from the cribriform plate. It is continuous below with the bony nasal septum and anchors the anterior end of the falx cerebri.
MIDDLE CRANIAL FOSSA:
The important features including the foramen rotundum, foramen ovale and foramen spinosum and the sphenoid body are addressed above.
POSTERIOR CRANIAL FOSSA:
The internal acoustic acoustic meatus is a shallow foramen in the posterior slope of the petrous ridge of the temporal bone. It is here that the Facial (CN VII) and Vestibulocochlear nerves (CN VIII) pass to their respective targets within the petrous temporal bone.