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4.2.6 Openings in the base of the cranium

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(3.57)

Now we’ll move on, to take a look at the openings in the floor of the anterior and middle cranial fossa that we saw earlier. We’ll look at three openings that pass forwards, two openings that pass downwards, and one that, in spite of appearances, passes obliquely backwards. We’ll start with the ones that pass forwards.

This round opening just in front of the anterior clinoid process is the optic canal, for the optic nerve. Lateral to it, this large triangular opening is the superior orbital fissure, which we’ve seen already. Numerous nerves and blood vessels pass through it into the orbit.

Below and behind the medial end of the superior orbital fissure, this smaller round opening, the foramen rotundum, is for the maxillary branch of the trigeminal nerve. We’ll put this pointer in the foramen rotundum, and go round to the outside. Here’s the superior orbital fissure again. Here, medial to it, is the optic canal. The foramen rotundum emerges not into the orbit, but into the ptreygo-maxillary fissure.

The two openings that pass downward are the foramen ovale, for the mandibular branch of the trigeminal nerve, and just behind and lateral to it, the foramen spinosum, for the middle meningeal artery.

To see where those two come out we’ll go round to the underside. Here’s the foramen ovale, just behind the lateral pterygoid plate. Here's the foramen spinosum, just behind and lateral to the foramen ovale.

The last opening to look at is this untidy looking opening, the foramen lacerum, for the internal carotid artery. In a dry skull the appearance of the foramen lacerum is quite misleading: it appears to pass straight down through the base of the skull emerging here, at the tip of the petrous temporal bone.

In the living body the apparent opening ...

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(3.57)

Now we’ll move on, to take a look at the openings in the floor of the anterior and middle cranial fossa that we saw earlier. We’ll look at three openings that pass forwards, two openings that pass downwards, and one that, in spite of appearances, passes obliquely backwards. We’ll start with the ones that pass forwards.

This round opening just in front of the anterior clinoid process is the optic canal, for the optic nerve. Lateral to it, this large triangular opening is the superior orbital fissure, which we’ve seen already. Numerous nerves and blood vessels pass through it into the orbit.

Below and behind the medial end of the superior orbital fissure, this smaller round opening, the foramen rotundum, is for the maxillary branch of the trigeminal nerve. We’ll put this pointer in the foramen rotundum, and go round to the outside. Here’s the superior orbital fissure again. Here, medial to it, is the optic canal. The foramen rotundum emerges not into the orbit, but into the ptreygo-maxillary fissure.

The two openings that pass downward are the foramen ovale, for the mandibular branch of the trigeminal nerve, and just behind and lateral to it, the foramen spinosum, for the middle meningeal artery.

To see where those two come out we’ll go round to the underside. Here’s the foramen ovale, just behind the lateral pterygoid plate. Here's the foramen spinosum, just behind and lateral to the foramen ovale.

The last opening to look at is this untidy looking opening, the foramen lacerum, for the internal carotid artery. In a dry skull the appearance of the foramen lacerum is quite misleading: it appears to pass straight down through the base of the skull emerging here, at the tip of the petrous temporal bone.

In the living body the apparent opening on the underside is filled in with dense cartilage and fibrous tissue, represented by this material. Fibrous tissue also fills in this ragged part of the internal bony opening.

What’s left of the foramen lacerum is a clean cut opening, through which the internal carotid artery emerges from its obliquely runnning bony tunnel, the carotid canal. The other end of the carotid canal, as we saw in the previous section, is back here, just medial to the styloid process.

In front of the opening for the carotid canal there’s one further opening that we haven’t seen yet, the opening for the auditory tube, also called the eustachian tube. The auditory tube passes backwards and laterally, to emerge here in the middle ear.

The auditory tube is longer than this: medially it’s prolonged by a tube of cartilage, represented by this colored material. The auditory tube opens into the nasopharynx, as we’ll see.

We’ll look at these openings again, in the sections of these two tapes that deal with the blood vessels and cranial nerves.

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