Topography within the corpus callosum

It is generally assumed that there is a topographical arrangement of axons within the corpus callosum according to their origin. However, actual anatomical evidence from human studies is very limited and sometimes contradictory. Callosal axons from the occipital cortex cross in the splenium, mostly in its lower part; the degeneration of this pathway has been shown after small lesions with the Weigert-Pal stain (Dejerine and Dejerine-Klumpke, 1895; Van Valkenburg, 1908) and with the Nauta method (Clarke and Miklossy, 1990). Callosal axons from the anterior parts of the frontal lobe cross within the genu as demonstrated with the Glees method in a case of frontal leucotomy (Beck, Meyer, and Le Beau, 1951). Several attempts have been made to trace callosal pathways from the temporal lobe using myelin stains after localized lesions (Zingerle, 1912; Van Buren and Yakovlev, 1959); the callosal bundles could not be traced to the midsagittal plane, probably owing to their "dilution" with other, nonaffected fiber bundles (for discussion, see Clarke et al., 1995). Axons from the posterior parietal and posterior temporal cortex have been reported to cross in the splenium and possibly also in the posterior part of the body of the corpus callosum, those from the lower frontal and anterior parietal convexity in the genu and those from the upper frontal and anterior parietal convexity in the anterior two thirds of the body; relatively discrete signs of Wallerian degeneration (my-

elin pallor, loss of fibers, presence of phagocytes) were observed there after lesions in the corresponding parts of the hemispheres (De Lacoste, Kirkpatrick, and Ross, 1985). Thus, the genu and the splenium, but not the body, are likely to contain relatively tight bundles of axons originating in restricted cortical regions and arranged topographically according to their origin. However, none of the published observations can affirm that a given bundle in the splenium or genu contains all cal-losal fibers from a given cortical region.

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