Last's Anatomy: Regional and Applied

Part twenty-one. Cranial fossae

Anterior cranial fossa

The anterior cranial fossa is formed by the frontal bone, the cribriform plate of the ethmoid, the lesser wings and anterior part of the body of the sphenoid. The floor of the fossa roofs in the orbits, ethmoidal sinuses and the nose.

The gyri and sulci of the undersurface of the frontal lobes cause grooves and ridges on the orbital parts of the frontal bone. From the nose up to 20 olfactory nerve filaments (on each side) perforate the dura and arachnoid mater over the cribriform plate and pass upwards through the subarachnoid space to enter the olfactory bulb, from which the olfactory tract passes back on the inferior surface of the frontal lobe (see Fig. 7.4, p. 459).

The anterior ethmoidal nerve and artery have a very short course in the fossa. Coming in from the orbit they lie beneath the dura at the edge of the cribriform plate, where they run forwards in a slit and pass down into the nose at the side of the crista galli.

The posterior border of the lesser wing of the sphenoid lies at the boundary between the anterior and middle cranial fossae. The sphenoparietal sinus runs between the two layers of the dura at this edge, which fits into the stem of the lateral cerebral sulcus (see p. 456); a branch of the middle meningeal artery crosses over the sinus to reach the anterior cranial fossa.

Middle cranial fossa

The middle cranial fossa consists of a median part, formed by the sphenoid body, and right and left lateral parts, formed by the cerebral surfaces of the greater wings of the sphenoid and the squamous and petrous parts of the temporal bones. The median part (Fig. 6.102) contains the pituitary gland, optic nerves and chiasma, and the intercavernous sinuses. The right and left lateral parts each contain the cavernous sinus, the third to the sixth cranial nerves and trigeminal ganglion, internal carotid artery, middle and accessory meningeal vessels, and the greater and lesser petrosal nerves. The temporal lobes lie on the right and left lateral parts.

The superior border of the petrous temporal bone lies at the boundary between the middle and posterior cranial fossae; the superior petrosal sinus lies here between the two layers of the dura.

Pituitary gland

The pituitary gland (hypophysis cerebri) lies in its fossa (sella turcica) with the diaphragma sellae above it. Intercavernous sinuses lie in the diaphragma and in the dural floor of the fossa. On each side is a flange of dura mater separating the gland from the upper part of the cavernous sinus. The pia and arachnoid blend with the capsule of the gland. Below the fossa lies the body of the sphenoid bone containing the sphenoidal air sinuses. When small the sinuses lie anteroinferior to the fossa, but when large one or both extend back beneath the fossa.

The pituitary stalk slopes downwards and forwards to the perforation in the diaphragma, connecting the posterior part of the gland to the hypothalamus (see p. 470). The upper, or infundibular, part of the stalk is hollowed out (continuous with the third ventricle). The optic chiasma lies above the diaphragma sellae, anterosuperior to the pituitary stalk. A pituitary tumour, rising upwards, presses on the lower anterior part of the chiasma and the medial sides of the optic nerves (so causing hemianopia of the temporal fields; see p. 500).

Structure. The gland is a composite structure consisting essentially of two main parts, the anterior and posterior lobes; these terms are not entirely synonymous with the terms adenohypophysis and neurohypophysis, which indicate different developmental origins. The adenohypophysis, developed from an ectodermal saccule (Rathke's pouch) of the stomodeum (see p. 24), consists of the pars anterior (distalis), pars tuberalis and the rudimentary pars intermedia. The neurohypophysis, developed as a neuroectodermal downgrowth from the floor of the third ventricle, consists of the pars posterior (nervosa), the infundibular stalk and the median eminence (see p. 470). The pars anterior and the pars posterior form the anterior and posterior lobes respectively.

The pars anterior accounts for 75% of the whole gland. It is highly vascular, and its cells consist of 50% chromophobes, 35% acidophils and 15% basophils. The acidophils (eosinophils) secrete growth hormone and prolactin, with the basophils producing ACTH, TSH, FSH and LH (adrenocorticotropic, thyroid-stimulating, follicle-stimulating and luteinizing hormones). The pars tuberalis is a small extension of the pars anterior along the pituitary stalk, but its cells are different and their function not known. The pars anterior lies in front of a narrow cleft (the remains of the pharyngeal downgrowth), behind which is the small pars intermedia. It consists of a number of colloid-filled vesicles and secretes MSH (melanocyte-stimulating hormone). Its cells contain endorphins.

The pars posterior consists largely of about 100000 unmyelinated nerve fibres whose cell bodies are in the supraoptic and paraventricular nuclei of the hypothalamus. Neurosecretory material manufactured in the cell bodies (mainly oxytocin from the paraventricular cells and vasopressin—antidiuretic hormone—from the supraoptic cells) migrates at the rate of about 3mm/day along their axons which run in the pituitary stalk and end in the pars nervosa. Both hormones are combined with a carrier protein, neurophysin. Scattered among the nerve fibres are the rather scanty pituicytes which resemble astrocytes.

Blood supply and the hypophyseal portal system. A single inferior and several superior hypophyseal arteries arise from the internal carotid artery. Venous blood enters the adjacent cavernous and intercavernous sinuses. In addition to these vessels there is a hypothalamo-hypophyseal portal venous system connecting the hypothalamus and neurohypophysis with the adenohypophysis. These fine vessels form a plexus on the stalk and pars anterior. Into them in the upper part of the stalk are secreted the various hypophysiotropic hormones (hormone-releasing factors) derived from cells of the median eminence and possibly other parts of the hypothalamus. The portal vessels then transport these substances to their target cells which are the chromophils of the anterior lobe. Thus the anterior lobe cells are controlled by hypothalamic messengers delivered to them by a portal venous system. Reverse blood flow in this system and venous drainage from the hypophysis to the hypothalamus has also been suggested. There is no blood–brain barrier (see p. 472) in the posterior pituitary or median eminence.

Surgical approach. The gland can be approached through the anterior cranial fossa by elevating the frontal lobe of the brain, the subfrontal approach, or by the trans-sphenoidal approach, entering the pituitary fossa from below through a sphenoidal air sinus. This is entered either via the ethmoidal air cells after raising the periosteum from the medial wall of the orbit, or by elevating the nasal mucosa from the nasal septum and removing the septum (the nasal cavity itself is not entered).

The internal carotid artery emerges from the roof of the cavernous sinus medial to the anterior clinoid process and curves immediately backwards, lying on the roof of the sinus before curving upwards lateral to the optic chiasma. At the anterior perforated substance it divides into its terminal branches (Fig. 6.106). The curve of the internal carotid artery in and above the cavernous sinus as seen in a lateral carotid arteriogram (like a U on its side, opening backwards) is commonly called the carotid siphon (Fig. 6.107).

The ophthalmic artery branches from the internal carotid immediately above the roof of the cavernous sinus. The internal carotid arteries come from below and laterally, the optic nerves come from above and medially (i.e. from the chiasma), and this is the relationship of nerve and artery in the optic canal (Fig. 6.53).

The optic nerve passes forward, down and laterally from the chiasma to the optic canal. Clad only in pia mater, it receives its sleeve of arachnoid and dura mater at the optic canal. Its intracranial part, in the chiasmatic cistern, is supplied by branches of the anterior cerebral artery that run down from the chiasma.

The oculomotor nerve leaves the medial side of the crus of the cerebral peduncle (see Fig. 7.18, p. 475). The nerve passes forwards between the posterior cerebral and superior cerebellar arteries (see Fig. 7.19, p. 475), just below the free margin of the tentorium cerebelli, below the posterior communicating artery; aneurysms may damage the nerve here. It crosses the interpeduncular cistern and enters the roof of the cavernous sinus in the middle fossa.

The trochlear nerve, after emerging from the dorsal surface of the brainstem below the inferior colliculus (see Fig. 7.19, p. 475), curls around the cerebral peduncle and runs forward between the posterior cerebral and the superior cerebellar arteries, lateral to the oculomotor nerve in the interpeduncular cistern, just below the free margin of the tentorium cerebelli (i.e. in the posterior fossa). It enters the middle fossa just behind the oculomotor nerve and enters the roof of the cavernous sinus, near where the free and attached margins of the tentorium cerebelli cross each other.

The abducens nerve enters the middle cranial fossa by passing over the apex of the petrous temporal and then runs round the lateral side of the ascending part of the internal carotid artery (Fig. 6.105) as it enters the cavernous sinus.

The posterior communicating artery, joining the internal carotid and posterior cerebral artery in the circle of Willis, lies in the interpeduncular cistern, above and lateral to the pituitary gland (see Fig. 7.19, p. 475).

Trigeminal ganglion

The trigeminal ganglion lies beneath the dura mater in the floor of the middle cranial fossa alongside the cavernous sinus, and occupies the trigeminal impression, a small fossa on the front of the apex of the petrous temporal bone posterolateral to the foramen lacerum (Figs 6.101 and 8.4, p. 529). The trigeminal nerve leaves the pons in the posterior fossa and runs forwards to cross the upper border of the petrous bone, upon which it leaves a shallow groove some 5mm wide; it is accompanied by the small motor root, which lies below the sensory root. They pass beneath the superior petrosal sinus at this point (Fig. 6.105). As they do so, with their covering of pia, they evaginate a diverticulum of the inner layer of the dura and the arachnoid, the trigeminal cave (of Meckel), which passes forwards with them to lie in the trigeminal impression beneath the dural floor of the middle cranial fossa, the sensory root now enlarged to form the ganglion, with the motor root still below it. The evaginated inner layer of dura and the arachnoid fuse with the pia mater at the middle of the trigeminal ganglion, so the sensory and motor roots of the nerve and the posterior half of the ganglion are bathed in cerebrospinal fluid (Fig. 6.104).

Blood supply. The accessory meningeal artery enters through the foramen ovale and runs up along the mandibular division to reach the ganglion, a similar arrangement to that in the spinal nerves. Small ganglionic branches are also given off by the internal carotid artery in the cavernous sinus.

The surgical approach to the sensory root and posterior half of the ganglion, in Meckel's cave, traverses the subarachnoid space. The anterior half of the ganglion and the three divisions of the trigeminal nerve lie in front of Meckel's cave. The upper part of the anterior half of the ganglion, with the ophthalmic and maxillary divisions, lies in the fibrous lateral wall of the cavernous sinus; the lower part of the ganglion and the mandibular division lie in the middle fossa between the otherwise fused two layers of dura mater. An extradural approach is therefore possible across the floor of the middle fossa by stripping the dura from the bone, avoiding entry into the subarachnoid space.

Transient facial palsy sometimes follows the latter approach. The explanation is thought to be that in stripping up the dura from the floor of the middle fossa tension is exerted on the greater petrosal nerve (which lies below the trigeminal ganglion; see below) and therefore on the geniculate ganglion. Subsequent oedema here causes pressure on the motor fibres of the facial nerve with paralysis of the muscles until the oedema subsides and the nerve recovers.

The mandibular division of the trigeminal nerve passes laterally to descend through the foramen ovale (Fig. 6.101). It is joined just below the foramen ovale by the small motor root to form the (mixed) mandibular nerve, a similar arrangement to that of the spinal nerves in the intervertebral foramina.

The maxillary nerve passes forwards within the inner layer of dura to leave the skull through the foramen rotundum (Fig. 6.102). The branches of the ophthalmic nerve pass forwards likewise, to leave the skull through the medial end of the superior orbital fissure.

The greater petrosal nerve (from the nervus intermedius part of the facial, see p. 498) emerges from its hiatus in the petrous bone (see Fig. 8.4, p. 507) and runs obliquely forwards, between the two layers of the dura mater and beneath the trigeminal ganglion to the foramen lacerum. Here it is joined by the deep petrosal nerve, a branch from the carotid plexus of sympathetic nerves. The two join to form the nerve of the pterygoid canal (Vidian nerve). This nerve enters the posterior end of the pterygoid canal at the front of the foramen lacerum and runs along the canal, through the body of the sphenoid bone, to join the pterygopalatine ganglion (see p. 370).

The lesser petrosal nerve (from the glossopharyngeal) leaves its hiatus in the petrous bone (see Fig. 8.4, p. 507) and runs forwards, lateral to the greater petrosal nerve, between the two layers of the dura mater to emerge through the foramen ovale to join the otic ganglion (see p. 366).

The middle meningeal artery is considered on page 442.

Posterior cranial fossa

The posterior cranial fossa lies behind the clivus (see p. 510) and in front of the squamous part of the occipital bone which extends laterally to where the fossa is bounded anterolaterally by the petrous and mastoid parts of the temporal bone. It lodges the convexities of the cerebellar hemispheres as well as the pons and medulla oblongata. In it the fifth to twelfth cranial nerves inclusive pierce the dura mater (Fig. 6.109).

The trigeminal nerve leaves the middle of the anterolateral surface of the pons by two roots, a large sensory and a small motor (see Fig. 7.18, p. 475). They lie close together. The motor root emerges somewhat above and medial to the sensory root, but spirals to enter the mouth of Meckel's cave below it. The sensory root itself shows a spiral arrangement of its fibres. At the junction with the pons the mandibular fibres lie superior, the ophthalmic inferior with the maxillary fibres between, but in Meckel's cave the mandibular fibres lie most laterally and the ophthalmic fibres most medially.

The abducens nerve leaves the brainstem near the ventral midline at the junction of the pons and the pyramid of the medulla (see Fig. 7.18, p. 475) and runs upwards through the pontine cistern. It enters the dura mater of the clivus some distance above its origin from the brainstem, and runs thence upwards between the two layers of the dura, passing over the apex of the petrous bone, to enter the posterior end of the cavernous sinus (see also p. 445). The relatively long intracranial course of this delicate nerve renders it particularly vulnerable to increase of intracranial pressure; paralysis of the lateral rectus may be an early sign in such cases.

The facial and vestibulocochlear nerves, with the intervening nervus intermedius part of the facial nerve, leave the lateral end of the junction of pons and medulla (see Fig. 7.6, p. 461) and pass upwards to enter the internal acoustic meatus, accompanied by the labyrinthine artery. This vessel is a branch of the basilar artery, or arises from the anterior inferior cerebellar artery.

The internal acoustic meatus is a foramen directed laterally in the posterior surface of the obliquely set petrous bone. Its fundus consists of a plate of bone divided by a horizontal crest into an upper and lower semicircle (Fig. 6.108). The facial nerve and its nervus intermedius part pierce the front of the upper part, the cochlear nerve the front of the lower part (by many branches in spiral arrangement). The vestibular nerve pierces the plate posteriorly, by upper and lower divisions that lie behind the facial nerve foramen (separated by a vertical bar of bone) and the spiral cochlear foramina respectively. Each division of the vestibular nerve is connected to the vestibular ganglion deep in the meatus. Behind the vestibular area is a single foramen (the foramen singulare) for the passage of the branch of the inferior division to the posterior semicircular duct. The labyrinthine artery divides in the meatus and its branches accompany the nerves through the bony plate.

The subarcuate fossa lies lateral to the internal acoustic meatus (Fig. 6.105). It is a very shallow fossa against which the flocculus of the cerebellum lies.

Further laterally on the posterior surface of the petrous bone is the orifice of the aqueduct of the vestibule, a narrow slit overhung by a sharp scale of bone (Fig. 6.105). The endolymphatic sac (see p. 421) hangs down from this slit beneath the inner layer of the dura.

The glossopharyngeal, vagus and accessory nerves arise from the side of the medulla oblongata as a series of rootlets lying vertically between the olive and the inferior cerebellar peduncle (see Fig. 7.6, p. 461). The three nerves run laterally across the occipital bone and pass through the jugular foramen (Fig. 6.109).

The spinal root of the accessory nerve enters the posterior fossa through the foramen magnum (Fig. 6.109). It arises by a series of rootlets that emerge from the lateral surface of the upper five or six segments of the cervical cord posterior to the denticulate ligament. These rootlets unite into a single trunk that passes forwards over the top of the ligament and ascends lateral to the vertebral artery to unite with the cranial root medial to the jugular foramen.

The jugular foramen is divided by two transverse septa of the inner layer of dura into three compartments. These septa may ossify. The glossopharyngeal nerve and inferior petrosal sinus share the anterior compartment, vagus and accessory nerves lie in the middle compartment, while the large posterior compartment is occupied by the termination of the sigmoid sinus. The inferior border of the petrous bone shows a deep notch immediately below the internal acoustic meatus (Fig. 6.105). This notch is indented by the inferior ganglion of the glossopharyngeal nerve; the aqueduct of the cochlea (see p. 419) opens into the depths of the notch and by this means perilymph drains into the subarachnoid space. The groove made by the inferior petrosal sinus is seen to enter the jugular foramen medial to the notch.

The hypoglossal nerve leaves the medulla by a vertical series of rootlets between the pyramid and the olive (see Fig. 7.18, p. 475). The rootlets unite into two roots which enter the hypoglossal canal separately, divided from each other by a septum of dura mater which occasionally ossifies.

The arteries in the posterior fossa comprise the two vertebral and the basilar arteries with their branches. After piercing the posterior atlanto-occipital membrane the vertebral arteries give off meningeal branches which enter the posterior fossa between the two layers of the dura mater at the foramen magnum (Fig. 6.109).

The vertebral artery then pierces the spinal dura mater and arachnoid and gives off the small posterior spinal artery (which may arise instead from the posterior inferior cerebellar branch). This fourth part of the vertebral artery runs forward in front of the highest denticulate ligament, in front of or between the rootlets of the hypoglossal nerve. It gives off the anterior spinal artery and the posterior inferior cerebellar artery and spirals up to meet its opposite fellow at the lower border of the pons to form the basilar artery. The posterior and anterior spinal arteries pass downwards through the foramen magnum on the spinal cord (see p. 491). The posterior inferior cerebellar artery is perhaps the most tortuous artery in the body. Its coils insinuate themselves between the rootlets of the hypoglossal, accessory and vagus nerves and the vessel is distributed to the cerebellum and medulla (see pp. 481 and 486).

The basilar artery runs up in front of the pons. It is not responsible for the ventral median groove in the pons; indeed, the artery is usually curved to one side of the midline (see Fig. 7.19, p. 475). It gives off the anterior inferior cerebellar artery and many pontine branches. The labyrinthine artery arises from the anterior inferior cerebellar or directly from the basilar trunk. The basilar artery ends at the upper border of the pons by dividing into the posterior cerebral arteries (see p. 472), immediately after giving off the superior cerebellar branches.