Page:Natural History Review (1861).djvu/226

214 the reader in the shape of a short account of the dissection which disclosed them.

The right cerebral hemisphere was removed down to the level of the corpus callosum, as seen in Fig. 4. At a point relatively much further distant from its posterior edge, 14, than is the case in man, we see the internal perpendicular fissure, 16. Posteriorly again to this fissure, and running nearly parallel with it, we see a second, 17, the "scissure des hippocampes" of M. Gratiolet. Corresponding with this indentation, we have within the cavity of the ventricle an eminence, 19, the lesser hippocampus, bounded by an arm or creek running up, 18, along its outer surface from the central ventricular expanse. This arm or creek was called, by another metaphor than those we have used, the third cornu of the lateral ventricle, in the phraseology of the old anatomists. The large smooth headland into which the hippocampus swells at 19, justifies the expression we find at page 19 of M. Gratiolet's work—"L' anfractuosité d' ergot . . . qui est plus evidente encore dans les Singes que dans l' Homme." In the brain of a cercopithecus now before us, its proportions are very much larger. The width of this third cornu was at its commencement three-eighths of an inch; and the similar cavity in a human brain examined at the same time was of the same width. But the cavity narrows much more rapidly in the orang than in man; and before reaching its termination, at a distance of one inch from its commencement, it becomes almost a linear cavity; but, as our figure shows, the distinctness of its limiting walls and the continuity of its lining membrane were unambiguously visible up to its very extremity. The length of this third cornu is as great absolutely, and relatively, therefore much greater in the cercopithecus, than in the orang. In the human brain it was but half an inch longer than in the orang, scooped out though it was in a posterior lobe relatively very much longer. Neither in the cercopithecus, nor in the orang, does the bourrelet or posterior rounded edge of the corpus callosum extend nearly so far back as to allow us to take it as "the best measure of the position" of the third cornu; indeed, when we find Tiedemann speaking of the pedes hippocampi minores as "Processus duo medullares qui a posteriore corporis callosi margine proficiscuntur," it is easy to understand how he came to overlook their existence altogether, "in cerebro Simiarum desunt," being so far in error as to their relations to neighbouring parts.

This relation of the posterior edge of the corpus callosum to the commencement of the third cornu is of importance, not merely as a guide to the discovery of that fissure, but also as, when coupled with the relations which the corpus callosum holds to the internal occipital figure 16, laterally, and to the corpora quadrigemina posteriorly, speaking unambiguously of great diminution of the antero-posterior diameter of the simious corpus callosum.