MODULE 11
The OPTIC TRACT
- - - Given all the excitement at the level of the chiasm, this next stretch of the pathway seems rather dull, but also a bit misleading. It seems as though the optic tract is running through the brain, but in fact it runs over the surface of the brain, and has been covered over by the expanding hemisphere. If you cut away the temporal lobe, as was done in Figure 2-14, it is clear that the optic tract is actually running over the surface of the cerebral peduncle to reach the lateral geniculate nucleus. The side view of Figure 2-16 shows this even more clearly. Returning to Figure 11-1, we see the this part of the visual pathway will be present in slides 34 - 26. As before, call up the slides using Figure 11-3. To follow the tract look at slides 34. 32, 30, 28 and 26.
- - - For some final views of the optic tract, see Figure 2-47. Then check it out on the brain models using Figure 2-39 and Figure 2-41 as guides.
- - - The fact of the matter is that the retinotopic organization of the visual pathway within the optic tract is of little clinical significance and most texts don't even mention it. The reason is that the optic tract is such a compact bundle of fibers that it is rare for only a part of it to be damaged. Thus, unlike the situation in the region of the visual radiation, lesions of the optic tract usually give rise to rather simple loss (a contralateral homonymous hemianopsia - lesion D in Blumenfeld's Figure 11.15). Never the less, we show the retinotopic organization of the tract just to give you a sense of continuity. Some would say that the fibers in the right tract gradually rotate counterclockwise by 90 degrees as the pass from the chiasm to the lateral geniculate, with the result that the most dorsal fibers near the chiasm (yellow) become the most medial ones as the tract approaches the geniculate. The most ventral ones (light blue) become the most lateral ones near the geniculate and the red and dark blue ones retain a central position.
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