Lab 2A
Cranial nerves, foraminae, orbit and eye

In this lab you will get to actually see the cranial foramina and the cranial nerves that pass through them. You’ll start by reviewing the cranial nerve stubs on the brain and where they enter the dura of the skull base, and then one student will peel the dura away from the inside of the skull base and look for the bony foramina that the cranial nerves pass through to exit the skull. At the same time, a second student will dissect the orbits and find all the small muscles, nerves and vessels that are associated with the eye. There is a lot of work to do, so divide and conquer is necessary, even though there is not much space to work.

Start by reviewing the stumps of the cranial nn. on the brain. Some are large and obvious, but others are small, or are made up of many small cranial nervlets on the brain. You may as well go in order from I to XII.

CN I, Olfactory tract and bulb - lie on the inferior surface of the frontal lobe of the cerebrum. The actual olfactory n. is made up of many small olfactory nervelets that pass through the cribriform plate and synapse with secondary sensory neurons that make up the optic bulb. The olfactory tract is the axons of those secondary sensory neurons.

CN II, Optic tract - cross at the optic chiasm. These are the axons of third-order sensory neurons. The first-order neurons of the optic system are in the retina.

CN III, Oculomotor n. - emerge from the brainstem between the posterior cerebral and superior cerebellar aa.

CN IV, Trochlear n. - the only CN that exits the brainstem from its dorsal surface.

CN V, Trigeminal n. - a large CN that exits just lateral to the pons.

CN VI, Abducent n. - small, exits the brainstem close to the midline at the pontomedullary junction.

Cranial Fossae:

Now we’ll turn to the inside of the skull base, and look for the CN stubs where they enter the dura mater and where they exit the skull. The base of the skull is divided by bony landmarks into 3 cranial fossae, and each of those fossae contain some number of foramina that the cranial nerves exit.

It’s important to keep in mind that where the CNs pierce the dura (where they disappear from sight) is not necessarily where they exit the skull. This is illustrated in figure 4.6. Sometimes the CN’s run between the two layers of dura for some distance before they reach the foramina they exit the skull from. You should be able to identify the foraminae of the skull both in a dry skull and in the cadaver’s skull.

Now review the definitions of the cranial fossae (no dissection required):

The anterior cranial fossa is formed mostly by an extension of the frontal bone, the orbital plate, and is bordered posteriorly by the lesser wing of the sphenoid bone. In the midline, part of the ethmoid bone sits within the frontal bone. The cristal galli (attachment site for falx cerebri) and the cribriform plate (openings for olfactory nervelets) are part of the ethmoid bone. Therefore CN I is the only CN that passes through the anterior cranial fossa, specifically through the holes in the cribriform plate.

The middle cranial fossa contains many of the cranial foramina, which are concentrated in the greater wing of the sphenoid bone. The middle fossa consists of the greater wing of the sphendoid bone the squamous part of the temporal bone, and the anterior region of the petrous part of the temporal bone.

The posterior cranial fossa consists of the posterior part of the petrous part of the temporal bone and the occipital bone.

One student should now work on peeling the dura from the middle and posterior cranial fossae while the other starts on the orbit. It is hard to peel the dura off, so do your best to reveal the following nerves and foramina.

Just medial to the anterior clinoid process of the lesser wing of the sphenoid you’ll find the optic canal, which allows passage of the optic nerve tract and the ophthalmic a. into the orbit.

Just inferior to the greater wing of the sphenoid, sitting between the greater and lesser wings, is an elongate slit, the superior orbital fissure. The superior orbital fissure allows passage of CN III, CN IV, CN V1, and CN VI into the orbit. The ophthalmic v., and sympathetic neurons also pass through the superior orbital fissure. All of these nerves pass through the cavernous sinus posterior to the superior orbital fissure. Carefully peel the dura off the cavernous sinus to see the

Just inferior to the superior orbital fissure is the foramen rotundum, which allows CN V2 to pass out to the upper jaw (maxilla, thus the maxillary n.). CN V2 will be visible in the opened cavernous sinus.

Posterior and lateral to the foramen rotundum is the foramen ovale, which allows passage of CN V3, also visible in the opened cavernous sinus.

Lateral to the foramen ovale is the foramen spinosum, which allows passage of the middle meningeal a. into the skull. The foramen spinosum is the origin point of the skull impressions left by the middle mengingeal a.

In a dry skull, look just lateral to the body of the sphenoid bone and you’ll see the openings into the carotid canal, which allow the internal carotid a. to enter the cranial cavity.

Between the temporal and occipital bone is an irregularly shaped “foramen” called the foramen lacerum. In life the foramen lacerum is covered by heavy connective tissue, so it is only a foramen in the dry skull when that connective tissue has been removed.

The posterior cranial fossa consists of the posterior part of the petrous part of the temporal bone, and the occipital bone. The most obvious feature of the posterior fossa is the foramen magnum. The spinal cord, vertebral aa., and CN XI pass through the foramen magnum. Recall that CN XI passes through the foramen magnum to enter the cranial cavity, not to exit the cranial cavity.

Just anterior to the foramen magnum are the paired hypoglossal canals that CN XII passes through on its way to the tongue.

The jugular foramen sits between the occipital and temporal bones. This is where the sigmoid sinuses become the internal jugular vv., and also where CN IX, X, and XI pass out of the skull.

Finally, in the petrous part of the temporal bone is the internal auditory (= internal acoustic) meatus, where CN VII and CN VIII pass out of the cranial cavity and into the temporal bone.

A quick inventory of the contents of the orbit includes:

• Eyeball

• CN II

• CN III and the ciliary ganglion (para. part of CN III)

• CN IV

• CN V1

• CN VI

• Ophthalmic a. and its branches

• Ophthalmic vv.

• Lacrimal gland

• 7 muscles that move the eyelid (1 muscle) and eyeball (6 muscles, collectively called the extra-ocular muscles)

After removing the bone that forms the roof of the orbit you’ll have to pick through the orbital fat pad. The more superior structures in the orbit are:

The frontal n. (V1), which will split to form the supratrochlear n. and supraorbital n. Recall these two sensory branches of V1 that you looked for during dissection of the face. The nasociliary n. is another branch of the frontal n.

The levator palpebrae superioris m., which raises the upper eyelid.

The superior rectus m., one of the 6 extra-ocular muscles, lies just underneath the levator palpebrae superioris m.

The superior oblique m. is the most medial of the extra-ocular m. Follow it out toward the front of the orbit and find the trochlea, a pulley that the muscle tendon passes through that allows the muscle to produce a rotational movement of the eyeball.

The superior oblique muscle is innervated by CN IV, the Trochelear n. The nerve is small and enters the substance of the superior oblique m. near its origin from the sphenoid bone.

Opposite the trochlea, in the lateral anterior corner of the orbit, find the lacrimal gland (tear gland).

A small lacrimal n. (a branch of CN V1) runs out to the lacrimal gland. The main part of the lacrimal n. (the part from CN V1) is somatic sensory to the lacrimal gland. Along its course the lacrimal n. picks up sympathetic and parasympathetic nerve fibers that supply secretomotor (visceral motor) innervation to the lacrimal gland. Both sympathetic and parasympathetic stimulation result in tear production.

Branches of the ophthalmic a. include the supraorbital a. and the lacrimal a.

The ophthalmic v. has superior and inferior branches that unite at the back of the orbit and empty into the cavernous sinus.

To access structures in the more inferior part of the orbit, cut the levator palpebrae superioris m. and the superior rectus m. near their mid-point and reflect the cut ends superiorly.

You may find branches of the superior division of the oculomotor n. entering the bellies of the levator palbebrae superioris and the superior rectus mm.

The most lateral of the extra-ocular muscles is the lateral rectus m. You may be able to find CN VI entering the belly of the lateral rectus m. The medial rectus m. is the antagonist of the lateral rectus m.

You can’t miss the optic nerve tract entering the back of the eyeball.

You may also see multiple small nerve branches entering the back of the eyeball, the long ciliary nn. and the short ciliary nn.

• Both the long and short ciliary nn. contain somatic sensory fibers of CN V1 that provide somatic sensory innervation from the eyeball.

• Both long and short ciliary nn. also contain post-ganglionic sympathetic neurons from the superior cervical ganglion that traveled along with arteries. The sympathetic neurons innervate the smooth muscle in the inta-ocular blood vessels, the dilator pupillae m. (dilate the pupils), and the superior tarsal m., a smooth muscle in the upper eyelid that helps hold the eye open. The sympathetic neurons also cause relaxation of the ciliary m., which leads to the lens flattening out to facilitate far vision.

• The short ciliary nn. contain postganglionic parasympathetic neurons from CN III that synapsed in the ciliary ganglion. The only neurons that synapse in the ciliary ganglion are the pre- and postganglionic fibers of CN III. Somatic sensory and postganglionic sympathetic neurons pass through the ciliary ganglion but do not synapse there. The parasympathetic neurons innervate the sphincter pupillae m. (constricts the pupil) and cause contraction of the ciliary muscles, which allows the lens to thicken to facilitate near vision.

Inferior to the optic nerve tract you’ll also see small branches of CN III innervating extraocular mm. Move the eyeball around and look for the inferior oblique m. and the inferior recuts m., they will be the toughest to see. Feel free to transect the optic nerve tract to better see the inferior structures in the orbit.

Orbital Blow-out Fracture:

A blowout fracture of the orbit occurs when trauma causes increased pressure within the orbit and one or more of the walls of the orbit fractures. The most common causes are sports injuries and fisticuffs (or some combination of the two). Orbital material is often pushed into the paranasal sinus(es) due to the pressures involved. Although the medial wall of the orbit is thinnest, the floor of the orbit is most commonly fractured in a blow-out fracture. Fracture of the floor of the orbit may result in entrapment of the inferior rectus muscle, and a resultant deficit in upward gaze due to increased tension in the muscle.

Orbital Blow-out Fracture:

A blowout fracture of the orbit occurs when trauma causes increased pressure within the orbit and one or more of the walls of the orbit fractures. The most common causes are sports injuries and fisticuffs (or some combination of the two). Orbital material is often pushed into the paranasal sinus(es) due to the pressures involved. Although the medial wall of the orbit is thinnest, the floor of the orbit is most commonly fractured in a blow-out fracture. Fracture of the floor of the orbit may result in entrapment of the inferior rectus muscle, and a resultant deficit in upward gaze due to increased tension in the muscle.

Oculomotor Nerve Palsy (Neuropathy):

As its name implies, the oculomotor nerve supplies most of the muscles that move the eyeball, and so damage to this nerve leads to a general lack of normal eye movements, resulting in dipolpia (double vision).

A complete palsy (in which the entire nerve is knocked out) leads to the affected eye assuming the “down and out position”, in which the gaze is directed inferiorly and laterally. This is because the lateral rectus and superior oblique muscles are still innervated, and since their actions are unopposed they draw the gaze laterally (lateral rectus) and inferiorly (superior oblique). The oculomotor nerve also supplies the levator palpebrae superioris, so ptosis (drooping eyelid) is also typically present.

Parasympathetic nerve fibers that run in CN III are superficial and thus may be affected first by a compression neuropathy, before any movement disorder occurs, such that the first symptom may be mydriasis (dilated pupils, or a “blown pupil”) due to lack of innervation of the constrictor pupillae.

The causes of aquired third nerve palsy are diverse, and include space occupying lesions or tumors, inflammation and infection, trauma, demyelinating diseases such as Multiple Sclerosis, autoimmune disorders such as Myasthenia Gravis, cavernous sinus thrombosis, and any of a number of vascular disorders.

One of the vascular disorders than may lead to third nerve palsy is aneurysm, especially of the posterior communicating artery (PCA). The most common site of PCA aneurysm is where the PCA branches off the posterior cerebral artery. That point is typically right next to the oculomotor nerve, and so an aneurysm at that site may exert pressure on the nerve and lead to compression neuropathy.