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Let's talk about the third nerve.

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The cranial nerves are pretty small

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and that's why we have 3D,

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1 mm or sub 1 mm slice thickness,

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that we have reformatted from the axial into the coronal

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projection. And rather than using the reformat,

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I've put up the Sagittal 3D FLAIR,

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which shows the apparent exit of the third nerve from

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the interpeduncular cistern in the midbrain.

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You can see it quite well on the FLAIR,

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bathed by dark signal intensity fluid.

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If only it were always that easy.

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And it is when you have thin section imaging.

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Now, in the axial projection,

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let's start out by drawing the real origin of the

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oculomotor nerve, which is here and here.

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The oculomotor nerve comes off the third nerve nucleus

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and the adjacent Edinger-Westphal nucleus.

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Now, I'm not so much interested in you learning about the

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intramedullary origin of the nerve at this point,

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but I do want you to know that the third

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nerve has an exit in the midbrain.

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So it is coursing through the midbrain and then

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it comes out in the interpeduncular cistern.

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And it's a little hard to see as it exits.

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You can see it right there.

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I'm drawing over it right now,

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and I'm drawing over it again on the right side.

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But look at the long intramedullary intra-axial course

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in the brainstem of the third nerve.

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That is why the third nerve is so susceptible

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to compression in the midbrain,

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because it has a long longitudinal axis.

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Now, I'm going to take my arrows away or take my lines away,

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my orange lines away,

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and you can follow the third nerve

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as it continues onward.

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And its next stopping point is going to be the sulcus of

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the oculomotor nerve before it enters as one

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of the larger nerves, the cavernous sinus.

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So it goes interpeduncular cistern, oculomotor sulcus,

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and then into the cavernous sinus,

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where it then finds its way into the

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superior orbital fissure. Now,

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if we talk about the apparent origin

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of the oculomotor nerve,

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I'm talking about where the nerve comes out of the brain

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stem itself. When I'm talking about the real origin,

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I'm talking about where it starts out in the nuclei of

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the brainstem, namely in this case, the midbrain.

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Now, you can also see the third nerve

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in the coronal projection.

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An easy way to do it is just to cross-reference it.

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Here it is as a tiny little dot.

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Now, the third nerve comes out between

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the posterior cerebral artery.

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You can see the arc of the posterior cerebral artery on

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the right. It's a little harder to see on the left.

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It's right there.

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But right underneath it is the third nerve.

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So the third nerve is going to come out between the PCA

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posterior cerebral artery and the not-so

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well-seen superior cerebellar artery.

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You can see a wisp of it here on the right,

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but not so much on the left.

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This is important because aneurysms in this locus can

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produce paresis of the third nerve

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or so-called third nerve palsy.