NERVES RELATED TO THE ORBIT  

Cranial nerves III, IV, VI, and the Ophthalmic and Maxillary divisions of cranial nerve V all pass through the cavernous dural venous sinus on their way to the orbit. Here, in this restricted area, they are closely related to each other and to the plexus of postganglionic sympathetic fibers on the internal carotid artery. 

With the exception of the Maxillary division of the Trigeminal, these neural elements all enter the orbit through the superior orbital fissure. As they do so, they can be grouped according to their relationship with the annular tendon and the four rectus muscles that arise from it.

Therefore, the basic anatomy of both sinus and fissure has to be understood before the paths of the nerves can be studied effectively.

THE CAVERNOUS SINUS
(Click to open a reference figure for the following section)

The cavernous sinuses are venous spaces formed between the cranial (endosteal) and meningeal (investing) layers of dura mater on either side of the sella turcica (hypophysial fossa).

The sella turcica (The Turks Saddle) is the central landmark of the middle cranial fossa. This is the depression in the body of the sphenoid bone that holds the pituitary gland (hypophysis). It is bounded posteriorly by the dorsum sellae, a high transverse plate of bone projecting upward from the clivus, and anteriorly by the somewhat lower, rounded tuberculum sellae that separates it from the optic sulcus. The fossa is bounded laterally by the paired anterior and posterior clinoid processes.

The bony construct of the sella can be thought of as the framework of an old-fashioned four poster bed, with a canopy and side-curtains.

The body of the sphenoid is the bed itself, sloping down on either side to the floor of the middle cranial fossa.

The floor of the sella represents the mattress.

The clinoid processes represent the four posts of the bed, supporting the canopy.

These bony contours are closely followed by the cranial layer of dura, the periosteum of the inner surface of the skull.

The inner layer of the dura has a very different contour, however.

It stretches as a membrane (diaphragma sellae) above the sella turcica, held up by the clinoid processes, like the canopy of the bed.

From the margins of a central aperture, the diaphragm is evaginated downward into the fossa around the pituitary, blending with the capsule of the gland. This is as though a large pumpkin (the pituitary) had been dropped onto the canopy of the bed, forcing it down onto the mattress.

On either side, the investing layer of dura falls from the anterior and posterior clinoid processes and the lesser wing of the sphenoid to the floor of the middle cranial fossa, resembling the side curtains of the bed. These side curtains conceal:

    1. The apex of the petrous temporal bone.
    2. The cranial entrance of the carotid canal.
    3. The foramen rotundum.
    4. The cranial entrance to the superior orbital fissure.

The cavernous sinuses lie between the dural side curtains, the body of the sphenoid, and the pituitary gland and its capsule. They extend vertically from the floor of the middle cranial fossa to the diaphragma sella. Right and left cavernous sinuses are linked by intercavernous sinuses that cross the midline in front of and behind the pituitary gland.

“Cavernous” refers to the fact that the sinuses are trabeculated, forming a vascular meshwork reminiscent of the cavernous erectile tissues of the penis and clitoris. This means that blood flow through the sinus would be sluggish unless assisted by some means. The boost is provided by pulsations of the internal carotid artery, which enters the sinus from the carotid canal, passes forward, doubles back on itself under the overhang of the lesser wing of the sphenoid, and emerges through the roof of the sinus at the base of the optic canal. Here it gives off the ophthalmic artery, which accompanies the optic nerve through the optic canal to reach the orbit.

In its long travel through the cavernous sinus, the internal carotid artery is accompanied by the internal carotid plexus – postganglionic sympathetic fibers originating from cells of the superior cervical sympathetic ganglion.

Communications

The cavernous sinus is centrally located in the middle cranial fossa. It receives blood from cerebral “bridging” veins, the superior and inferior ophthalmic veins, and the sphenoparietal dural sinus along the free edge of the lesser wing of the sphenoid. Posteriorly it is drained by the superior and inferior petrosal dural sinuses.

The superior petrosal sinus runs posteriorly and laterally in a groove along the petrous ridge to join the junction of transverse and sigmoid sinuses.
The inferior petrosal sinus descends in a groove along the petrooccipital suture [check the terminology] to pass through the anterior part of the jugular foramen and end in the internal jugular vein.

Small emissary veins link the cavernous sinus with the pterygoid venous plexus in the infratemporal fossa. Most accompany the mandibular nerve through foramen ovale. A few penetrate the cartilage of the foramen lacerum.


[Alt. wording Plexiform emissary veins accompany the mandibular nerve through the foramen ovale and link the cavernous sinus with the pterygoid venous plexus in the infratemporal fossa.
In addition, few small veins leave the sinus and penetrate the cartilage of the foramen lacerum to reach the pterygoid plexus.]


THE SUPERIOR ORBITAL FISSURE AND THE OPTIC CANAL

The optic canal and the superior orbital fissure are the gateways between the orbit and the middle cranial fossa.

The optic canal opens directly into the apex of the orbit.

The superior orbital fissure opens into the rear of the orbit lateral to the optic canal. It is subdivided in life by an annular tendon (annulus tendoneus) that gives origin to four of the six extraocular muscles – the superior, inferior, lateral and medial recti, which pass directly forward to reach the globe of the eye. The tendon encompasses the expanded medial end of the superior orbital fissure as well as the orbital entrance of the optic canal.

As a rule, structures leaving the middle cranial fossa to reach the rectus muscles and/or the eye itself, will pass within the embrace of the annular tendon. All others will enter the orbit through the lateral part of the superior orbital fissure. Thus, Cranial Nerve II and the ophthalmic artery enter through the optic canal, within the annulus. Cranial Nerves III (Oculomotor), VI (Abducent), and the nasociliary branch of the ophthalmic division of Cranial Nerve V (Trigeminal) pass through the superior orbital fissure, within the annulus. Cranial Nerve IV (Trochlear), and the frontal and lacrimal branches of the ophthalmic division of the Trigeminal pass through the superior orbital fissure lateral to the annulus. So does the superior ophthalmic vein on its way back from the orbit to the cavernous dural venous sinus.

CRANIAL NERVE III (OCULOMOTOR)

Components:

The oculomotor nerve carries:

1. somatic motor (GSE) fibers for five of the seven extraocular muscles.

The two exceptions are the:

  1. superior oblique, whose tendon changes direction by passing through the trochlea (pulley), innervated by the Trochlear nerve (Cranial Nerve IV).

  2. lateral rectus, the abductor of the visual axis, innervated by the Abducent nerve (Cranial Nerve VI).

2. preganglionic parasympathetic motor (GVE) fibers destined for the ciliary ganglion.  These belong to the motor pathway controlling the constrictor pupillae and ciliaris muscles. They represent the efferent limb for the pupillary light and accommodation reflexes, respectively.

Course

The nerve emerges from the brainstem through the cerebral peduncle, at the upper border of the pons. It crosses the subarachnoid space, passing upward and forward between the superior cerebellar and posterior cerebral arteries, pierces the arachnoid membrane and grooves the upper surface of the posterior clinoid process. It then runs forward on [above?] the roof of the cavernous sinus toward the anterior clinoid process. As it approaches the superior orbital fissure, the oculomotor nerve descends through the roof of the sinus and passes beneath the anterior clinoid process. Here it gives rise to its superior and inferior divisions, which enter the orbit separately within the embrace of the annular tendon.

The superior division ascends into the upper part of the orbit to reach and supply first the superior rectus and then the voluntary portion of levator palpebrae superioris.

The inferior division gives rise to its three terminal branches in the apex of the orbit, lateral to the optic nerve. One passes beneath the optic nerve to reach the medial rectus. Another supplies the inferior rectus. The third and longest of the three branches, runs forward between the inferior and lateral rectus muscles to reach the inferior oblique.

The preganglionic parasympathetic fibers in the oculomotor nerve pass into the inferior division and the branch to the inferior oblique. They leave this nerve in the apex of the orbit and ascend lateral to the optic nerve as the motor root of the ciliary ganglion.

Communication:

As it passes through the roof and lateral wall of the cavernous sinus, the oculomotor nerve communicates with the ophthalmic division of the Trigeminal nerve and with the postganglionic sympathetic plexus on the internal carotid artery.

The extraocular muscle contain muscle spindles. Somatic afferent (GSA) fibers returning from these and other sensory endings leave the orbit in terminal branches of the oculomotor nerve. In the cavernous sinus most if not all are transferred to the ophthalmic nerve and reach the brainstem with the Trigeminal nerve.

Postganglionic sympathetic (GVE) fibers from the internal carotid plexus are distributed by the oculomotor nerve to blood vessels in the extraocular muscles. Some, however, pass into the superior division of the nerve and innervate the involuntary portion of the levator palpebrae superioris (superior tarsal muscle).

CRANIAL NERVE IV (TROCHLEAR)

Components:

The trochlear nerve carries:

  1. somatic motor (GSE) fibers that innervate the extraocular superior oblique muscle, whose tendon changes direction by passing through the trochlea, a cartilaginous pulley located anteriorly in the superomedial angle of the orbit.

Course:

The trochlear nerve is the only cranial nerve to emerge from the dorsal aspect of the brainstem. It appears immediately caudal to the inferior colliculus, hugs the surface of the brainstem as it winds ventrally, and passes between the superior cerebellar and posterior cerebral arteries, lateral to the much larger oculomotor nerve. It does not accompany the oculomotor nerve onto the roof of the cavernous sinus. Instead, it pierces the dura on the undersurface of the tentorium cerebelli behind the posterior clinoid process and runs forward in the lateral wall of the sinus, below the oculomotor nerve.

As they approach the superior orbital fissure, the two nerves change positions. The oculomotor descends to enter the orbit within the annular tendon, whereas the trochlear ascends to pass above and lateral to the tendon. The trochlear nerve then turns medially, passes above both superior rectus and levator palpebrae superioris and ends in the superior oblique muscle.

Communications:

As it passes forward in the lateral wall of the cavernous sinus, the trochlear nerve communicates with the ophthalmic division of the Trigeminal nerve and with the postganglionic sympathetic plexus on the internal carotid artery.

Somatic afferent (GSA) fibers returning from muscle spindles and other sensory endings in the superior oblique muscle leave the orbit in the trochlear nerve. Most if not all are transferred to the ophthalmic nerve in the cavernous sinus and reach the brainstem with the Trigeminal.

Postganglionic sympathetic (GVE) fibers from the internal carotid plexus are distributed by the trochlear nerve to blood vessels in the superior oblique muscle.

CRANIAL NERVE VI (ABDUCENT, or ABDUCENS)

Components:

The abducent nerve carries:

  1. somatic motor (GSE) fibers that innervate the extraocular lateral rectus muscle, which acts to abduct the visual axis of the eye.

Course:

The abducent nerve emerges into the posterior cranial fossa at the lower border of the pons, just below the anterior inferior cerebellar artery. It travels upward and forward either behind or in front of the artery, leaves the subarachnoid space, and pierces the dura on the clivus. The nerve reaches the ridge of the petrous temporal bone, turns forward in the angle between the superior and inferior petrosal sinuses, and enters the cavernous sinus lateral to the internal carotid artery. The nerve courses through the sinus between the artery and the lateral dural wall.

The abducent nerve passes through the superior orbital fissure within the annular tendon and below the oculomotor nerve. It runs forward on the ocular surface of the lateral rectus for a short distance and enters the muscle.

Communication:

As it traverses the cavernous sinus, the abducent nerve communicates with the ophthalmic division of the Trigeminal nerve and with the postganglionic sympathetic plexus on the internal carotid artery.

Somatic afferent (GSA) fibers returning from muscle spindles and other sensory endings in the lateral rectus muscle leave the orbit in the abducent nerve. Most if not all are transferred to the ophthalmic nerve in the cavernous sinus and reach the brainstem with the Trigeminal.

Postganglionic sympathetic (GVE) fibers from the internal carotid plexus are distributed by the abducent nerve to blood vessels in the lateral rectus muscle.