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4.8.10 Facial and vestibulo-cochlear nerves (cranial nerves VII, VIII)

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(4.56)

We'll move on now to look at cranial nerves seven through twelve. We'll begin with the seventh and eighth, the facial and vestibulo-cochlear nerves. As we've seen, these two nerves leave the brainstem just below the pons. Here's the facial, here's the vestibulo-cochlear.

To follow them we'll look at a dissection of the posterior cranial fossa, in which the cerebellum has been removed. Here's the back of the petrous temporal bone. Here are the facial and vestibulo-cochlear nerves. This is the vestibulo-cochlear nerve, this is the facial nerve. Together they pass through this opening, the internal auditory meatus.

Here's the internal auditory meatus in the dry skull. It's a short tunnel with three openings. The facial nerve passes forwards through this one, to enter its own bony tunnel, the facial canal. In the facial canal, the facial nerve has a complex course in the temporal bone, passing round the wall of the tympanic cavity, and coming out behind the styloid process, here.

To get a view of its course we've removed the front wall of the external auditory meatus along this line. We've also unroofed a small part of the petrous temporal bone, here. To represent the facial nerve we'll add this green wire to the picture.

Entering its canal the facial nerve passes forwards briefly, then makes an abrupt U-turn called the genu and passes backward. To follow it we'll come round to the outside and look into the tympanic cavity.

The facial nerve passes backwards high in the medial wall of the tympanic cavity, above the oval window. It then turns downwards, to emerge here at the stylomastoid foramen, just behind the root of the styloid process.

On its way through the temporal bone the facial nerve gives off three branches, the greater petrosal nerve which is ...

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(4.56)

We'll move on now to look at cranial nerves seven through twelve. We'll begin with the seventh and eighth, the facial and vestibulo-cochlear nerves. As we've seen, these two nerves leave the brainstem just below the pons. Here's the facial, here's the vestibulo-cochlear.

To follow them we'll look at a dissection of the posterior cranial fossa, in which the cerebellum has been removed. Here's the back of the petrous temporal bone. Here are the facial and vestibulo-cochlear nerves. This is the vestibulo-cochlear nerve, this is the facial nerve. Together they pass through this opening, the internal auditory meatus.

Here's the internal auditory meatus in the dry skull. It's a short tunnel with three openings. The facial nerve passes forwards through this one, to enter its own bony tunnel, the facial canal. In the facial canal, the facial nerve has a complex course in the temporal bone, passing round the wall of the tympanic cavity, and coming out behind the styloid process, here.

To get a view of its course we've removed the front wall of the external auditory meatus along this line. We've also unroofed a small part of the petrous temporal bone, here. To represent the facial nerve we'll add this green wire to the picture.

Entering its canal the facial nerve passes forwards briefly, then makes an abrupt U-turn called the genu and passes backward. To follow it we'll come round to the outside and look into the tympanic cavity.

The facial nerve passes backwards high in the medial wall of the tympanic cavity, above the oval window. It then turns downwards, to emerge here at the stylomastoid foramen, just behind the root of the styloid process.

On its way through the temporal bone the facial nerve gives off three branches, the greater petrosal nerve which is an autonomic branch, a branch in the middle ear to the stapedius muscle, and a special branch, the chorda tympani, represented by by this wire. The chorda tympani passes upwards on the inside of the tympanic membrane, and leaves the cranium here to join the lingual nerve.

Weve seen already that the chorda tympani transmits taste sensation from the anterior two thirds of the tongue. Now we'll follow the trunk of the facial nerve.

As we saw in the previous tape, the facial nerve passes through the parotid gland. In the dissection we'll look at now, the parotid gland gland has been completely removed. Here's the external auditory meatus, the sternocleidomastoid muscle, and the posterior belly of the digastric. Here's the styloid process.

Here's the trunk of the facial nerve, emerging from the stylomastoid foramen here. Now we'll add its branches to the picture. Between them they supply all the muscles of facial expression.

These temporal branches supply the frontalis muscle. These orbital branches supply the muscles around the eye, including orbicularis oculi. These buccal branches supply buccinator, orbicularis oris, and the muscles that move the upper lip.

These marginal mandibular branches, which pass just below the body of the mandible, supply the muscles that move the lower lip. They also supply the platysma.

In addition to supplying the muscles of facial expression, the facial nerve gives off this branch that supplies the posterior belly of the digastric, and the stylohyoid muscle.

Now that we've looked at the facial nerve, we'll look very briefly at the eighth cranial nerve, the vestibulo-cochlear. The vestibulo-cochlear nerve enters the internal auditory meatus, dividing as it does so into the vestibular and cochlear nerves.

The vestibular nerve passes through this opening into the inner ear, the cochlear nerve passes through this one. The two nerves transmit our senses of balance and hearing respectively.

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