(4.04)

In this section we'll look at the twelve cranial nerves, the sympathetic trunk, and the cervical nerves.

The cranial nerves are numbered by the order in which they leave the cranial cavity. Earlier in this tape we saw them emerging from the brain. In this section we'll follow the course of each nerve, look at its principal branches, and summarize its functions.

We'll begin with the first six cranial nerves. The first, the olfactory, and the second, the optic transmit our senses of smell and of eyesight. The third, the oculomotor, the fourth, the trochlear, and the sixth, the abducent, are motor nerves to the eye muscles; and the fifth, the trigeminal, is a large motor and sensory nerve to the face and jaws.

The first cranial nerve, the olfactory nerve is extremely short. It consists of a series of fine filaments which arise from the olfactory bulb on the underside of the frontal lobe. On each side the olfactory bulb lies here, just above the cribriform plate.

Here's a frontal section in the dry skull that goes through the cribriform plates: they're here. On each side the cribriform plate forms the narrow roof of the nasal cavity. Here's a medial view of the nasal cavity. The cribriform plate is here.

The filaments of the olfactory nerve, here they are in close-up, pass through the cribriform plate and run just beneath the mucous membrane to reach nerve endings in this olfactory area on the lateral and medial surfaces of the nasal cavity.

The next nerve we'll look at is the second cranial nerve, the optic nerve. We've seen the proximal ends of the optic nerves, emerging from the optic chiasm.

Here's the optic nerve, passing forward beneath the dura to enter the optic canal, which starts here. Here's the optic canal in the dry skull. Here on each side of the optic chiasm are the divided internal carotid arteries.

Just beneath the chiasm is the roof of the pituitary fossa: here's the divided stalk of the pituitary gland. To follow the optic nerve, we'll remove the roof of the orbit, leaving the optic canal intact. We'll remove this nerve, and the orbital fat, and these two muscles, which we'll see later.

Here's the optic nerve. It enters the orbit between the tendons of origin of the rectus muscles. It passes forwards and laterally, to enter the back of the eyeball.

Strictly speaking the optic nerve isn't a nerve, it's an extension of the brain. It's covered throughout its course by extensions of all three meningeal layers, dura, arachnoid and pia.

Here we've made a window in the dura surrounding the optic nerve. Here's the edge of the dura, here's the nerve itself, here's the arachnoid. The dura is continuous with the outer layer of the eyeball, the sclera. We'll be returning here shortly. For now we'll put the contents of the orbit back in place.

The optic chiasm is a cross-over point for optic nerve fibers. The fibers of each nerve that connect to the medial half of the retina cross over into the opposite optic tract. The fibers that connect to the lateral halves of the retinae stay on the same side.