Most claims about what the corpus callosum does are based on data about its size and shape. But how in the world can scientists produce accurate measurements of a structure as complex and irregularly shaped as the corpus callosum? Looked at from above, the CC resembles a raised topographical map (figure 5. i). A pair of ridges run oddly parallel for some distance, but diverge to the south. Flanking one ridge to the west and the other to the east lie plateaus, while a vast valley runs between the ridges. East-west striations traverse the entire territory. These striations—which represent millions of nerve fibers—constitute the corpus callosum.27As the ridges and valleys suggest, these fibers don’t run along a flat, two-dimensional surface; instead they rise and fall. Moreover, as the edges of the map indicate, the fibers are not wholly separate from other parts of the brain, but instead connect to and entangle with them. As one pair of researchers writes: ‘‘the corpus callosum is shaped much like a bird with complicated wing formation. Further these wings co-mingle with the ascending white matter tracts. . . making the lat-
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TABLE 5.1 Nineteenth — and Twentieth-Century Left/Right Brain Dichotomiesa
a. Taken from Harrington 198^. |
eral portion of the corpus callosum essentially impossible to define with certainty.28
Or one could imagine the CC as a bunch of transatlantic telephone cables. In the middle of the Atlantic (the valley on the map, which joins the left and right cerebral hemispheres), the cables are bundled. Sometimes the bundles bunch up into ridges; but as the cables splay out to homes and offices in North America and Europe, they lose their distinct form. Smaller bunches of wire veer off toward Scandinavia or the Low Countries, Italy or the Iberian Peninsula. These in turn subdivide, going to separate cities and ultimately to particular phone connections. At its connecting ends, the corpus callosum loses its structural definition, merging into the architecture of the cerebrum itself.
The ‘‘real’’ corpus callosum, then, is a structure that is difficult to separate from the rest of the brain, and so complex in its irregular three dimensions as to be unmeasurable. Thus, the neuroscientist who wants to study the CC must
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