Required Knowledge for the Base of Skull
- Pituitary fossa (function limited to named hormonal secretions – clinical effects not required)
- Cavernous sinus, Meckel’s cave, Rathke’s pouch, clivus
- All vascular and neural foramina for major vessels, CNs and their branches
The pituitary fossa lies in the central floor of the middle cranial fossa. The bony structures lining the fossa are all parts of the sphenoid bone. Anteriorly, the sphenoid bone forms the middle part of the fossa, with the middle clinoid processes on the lateral parts of the anterior surface. The floor of the fossa is the roof of the sphenoidal sinuses. The posterior wall is formed by the dorsum sellae, a vertical ridge of bone. The superolateral parts of the dorsum sellae extend as the posterior clinoid processes. The roof of the fossa is formed by a sheet of dura which extends from the middle to the posterior clinoid processes. The lateral walls are not covered with bone, but bounded by the cavernous sinus.
The pituitary gland is the structure that lies within the pituitary fossa. It is connected with the hypothalamus superiorly by the infundibulum. The pituitary gland is divided into the neurohypophysis (posterior lobe) and adenohypophysis (anterior lobe).
The neurohypophysis contains the axonal terminals of hypothalamic neurons. These release neuroendocrine hormones into the vasculature. These include:
- Anti-diuretic hormone (ADH)
The adenohypophysis is under hormonal control of the hypothalamus via the hypothalamo-pituitary portal system. Neuroendocrine hormones are released from axon terminals in the superior infundibulum and enter this system. The important hormones released by the adenohypophysis are:
- Growth hormone
- Follicle stimulating hormone and leutenising hormone (FSH/LH)
- Thyroid stimulating hormone (TSH)
- Adrenocorticotrophin (ACTH)
The pituitary gland is separated from the floor of the fossa by a venous plexus which drains into the circular sinus. It is covered laterally by dura which separates it from the cavernous sinus. The diaphragma sella, the roof of the pituitary fossa, contains a small hole for the infundibulum to pass through.
The pituitary fossa is inferior to the optic chiasm, hypothalamus and third ventricle. It is medial to the cavernous sinus and its contents. It is superior to the sphenoid sinuses and nasopharynx. The sphenoid sinus also lies anteriorly. The midbrain lies posteriorly.
The cavernous sinus lies in the middle cranial fossa, lateral to the pituitary fossa. It extends from the superior orbital fissure anteriorly to the apex of the petrous part of the temporal bone posteriorly (about 2 cm long). It is surrounded by dura or periosteum.
The cavernous sinus contains the cavernous part of the internal carotid artery, before it divides into the anterior and middle cerebral arteries. Coating this artery is the large internal carotid sympathetic plexus. The abducens nerve (VI) runs just lateral to the artery on the floor of the cavernous sinus.
From superior to inferiorly, the oculomotor (III), trochlear (IV), ophthalmic (V1) and maxillary (V2) nerves run in the lateral wall of the cavernous sinus. These nerves lie outside the dura coating the uncus of the temporal lobe. The posterior and inferior part of the sinus is indented by the trigeminal cave (Meckel’s cave) which contains the trigeminal ganglion.
The cavernous sinus drains the superior and inferior ophthalmic veins, the superficial medial cerebral vein, the inferior cerebral vein and the sphenoparietal sinus.
The cavernous sinus drains to:
- The transverse sinus via the superior petrosal sinus
- The internal jugular vein via the inferior petrosal sinus
- The pterygoid venous plexus via the emissary sphenoidal foramen, foramen lacerum and the foramen ovale
- The facial vein via the superior ophthalmic vein
The cavernous sinus is lateral to the pituitary fossa. It lies inferior to the optic chiasm / nerves / tracts and petro-sphenoidal ligament. The uncus of the temporal lobe is lateral. The trigeminal cave and ganglion lie posterior and inferior. The sphenoidal sinus is anterior.
Rathke’s pouch is a region ectoderm that, during embryological development, forms a cavity and migrates towards the developing neurohypophysis. Rathke’s pouch eventually forms the adenohypophysis.
Trigeminal cave (Meckel's Cave)
The trigeminal cave is a dural recess located lateral, posterior and slightly inferior to the cavernous sinus. It contains the trigeminal ganglion.
Anterior Cranial Foramina
The anterior cranial fossa has only a few foramina. These include:
- Foramen caecum, located in the frontal bone. It is present in 1% of the population and conveys the nasal emissary vein.
- Cribriform foramina extend through the ethmoid bone to the superior nasal cavities. They convey fibres of the olfactory nerve (I).
- Anterior and posterior ethmoidal foramina convey vessels and nerves of the same name between the anterior cranial fossa and the orbit (via the ethmoidal air cells).
Middle Cranial Foramina
The middle cranial fossa contains numerous foramina:
- The optic canals carry the optic nerves (II) and ophthalmic arteries.
- The superior orbital fissure conveys the oculomotor nerve (III), trochlear nerve (IV), ophthalmic nerve (V1) and the abducens nerve (VI). It also houses the ophthalmic veins and sympathetic branches to the eyeball and orbital vessels.
- The foramen rotundum transmits the maxillary nerve (V2) into the pterygopalatine fossa.
- The foramen ovale carries the mandibular nerve (V3) as well as parasympathetic fibres of the lesser petrosal nerve (IX). The accessory meningeal artery ascends through this foramen.
- The foramen spinosum conveys the middle meningeal artery and vein, as well as the recurrent meningeal branch of the mandibular nerve (V3)
- The foramen lacerum carries the deep petrosal nerve (parasympathetic fibres from the facial nerve to the pterygopalatine ganglion) as well as small vessels to the meninges
- The carotid foramen houses the carotid artery.
- A groove or hiatus for the greater petrosal nerve also carries the petrosal branch of the middle meningeal artery.
Posterior Cranial Foramina
The largest foramina are found in the posterior fossa.
- The foramen magnum is the largest of the cranium. The spinal cord, the ascending part of the spinal accessory nerve (XI), the vertebral arteries, dural veins and anterior/posterior spinal arteries all pass through this foramen.
- The jugular foramen is relatively large, and the glossopharyngeal nerve (IX), vagus nerve (X) and spinal accessory nerve (XI) exit the cranium through this route. The inferior petrosal and sigmoid sinuses unite and give rise to the internal jugular artery. Meningeal branches of the ascending pharyngeal and occipital arteries pass into the cranium through the jugular foramen.
- The internal acoustic meatus is at the boundary of the posterior and middle cranial fossae. It conveys the facial nerve (VII) and vestibulocochlear nerve (VIII). The facial nerve passes along a separate canal to exit at the styloid foramen.
- The hypoglossal canal is lateral and just above the foramen magnum, and carries the hypoglossal nerve (XII)
- The condylar canal carries an emissary vein from the sigmoid sinus to the vertebral veins of the neck. It is found just posterior to the occipital condyles.
- The mastoid foramen is found just posterior to the mastoid process, and carries the mastoid emissary vein from the sigmoid sinus and the meningeal branch of the occipital artery.
- 1: Head and Neck