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I want to talk about the real origin

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of the third cranial nerve.

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Not the origin where it comes out of the brainstem,

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but the origin of the nuclei in the brainstem.

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Now, I alluded to this in vignette one,

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but I want to get a little more detailed now.

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So, I'm going to draw.

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If I find my aqueduct of Sylvius,

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my third nerve nucleus and my accessory third nerve

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nucleus is going to be closer to the aqueduct of Sylvius,

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right in front of the periaqueductal gray,

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then the fourth nerve nucleus,

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which is a little farther anteriorly and a little further caudate.

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I gave you some tricks to find it.

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So, this nucleus closer to the aqueduct of Sylvius.

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And then, if we turn our attention

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to the T2-weighted image,

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where we can see the red nucleus right here and here.

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In fact, I'm going to make them red just for giggles.

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So one red on the right, one red on the left.

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Now I'm going to take my

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orange-colored oculomotor nerve or third nerve.

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And I'm going to watch it come out.

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And it bows a little bit.

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It almost bisects the red nucleus.

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Now, there are root filaments that contribute

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a little bit out to the side here,

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so you can get pathology on the lateral margin of the

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red nucleus that affects the oculomotor apparatus.

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But the main nerve takes a pretty long course

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intramedullary through the brainstem at

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the level of the superior colliculus.

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All of this being intramedullary and not easily

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visualized, certainly not visualized by the surgeon.

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And then, the nerve comes out in the interpeduncular

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cistern where it exits,

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headed for the oculomotor sulcus.

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There are two small dots representing the third nerve,

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sitting directly behind the mammillary bodies.

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Now, a couple of other salient points.

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When the third nerve comes out,

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as it exits in the interpeduncular cistern, it's mesial,

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or medial to the substantia nigra.

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So it's not going to come out over here.

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It may look like it does due to volume averaging,

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but it's going to come out always medial

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to the substantia nigra.

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So, that's another trick for locating its apparent

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exit from the midbrain.

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In the next vignette,

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I want to talk a little bit about what can happen to

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the third nerve and various syndromes in the medullary

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space of the midbrain, like Weber syndrome.

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Let's move on, shall we?