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Show 96 tion of the ventricular cavity extends into it, and is called tl1e " posterior cornu." In the lower and smaller forms of placental Mammals the surface of the cerebral hemispheres is either smooth or evenly rounded, or exhibits a very few grooves, which are technically termed 'sulci,' separating ridges or 'convolutions' of the substance of the brain; and the smaller species of all orders tend to a simpar smoothness of brain. But, in the higher orders, and especially the larger members of these orders, the grooves, or sulci, become extremely numerous, and the intermediate convolutions proportionately more c01nplicated in their meanderings, until, in the Elephant, the Porpoise, the higher A pes, and Man, the cerebral surface appears a perfect labyrinth of tortuous foldings. Where a posterior lobe exists and presents its customary cavity-the posterior cornu-it commonly happens that a particular sulcus appears upon the inner and under surface of the lobe, parallel with and beneath the floor of the cornuwhich is, as it were, arched over the roof of the sulcus. It is as if the groove had been formed by indenting the fioor of the posterior horn from without with a blunt instrument, so that the floor should rise as a con vex eminence. Now this eminence is what has been termed the 'Hippocampus minor;' the ' Hippocampus major' being a larger eminence in the floor of the descending cornu. What may be the functional importance of either of these structures we know not. As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between man and the apes, Nature has provided us, in the latter animals, with an almost complete series of gradations from brains little higher than that of a Rodent, to brains little lower than that of Man. And it is a remarkable circumstance, that though, so far as our present knowledge extends, there is one true structural break in the series of forms of Simian brains, this hiatus does not lie between Man and the man-like apes, but between the lower and the lowest Simians; or, in other words, between 97 the old ancl new world apes and monkeys, and the Lemurs. Every Lemur which has yet been examined, in fact, has its c~rebellum partially visible from above, and its posterior lobe, With the contained. posterior cornu and hippocampus minor, more or less rudimentary. Every Marmoset, American monkey, old :vorld 1nonkey, Baboon, or Man-like ape, on the contrary, has Its cerebeJlum entirely hidden, posteriorly, by the cerebral lobes, and possesses a large posterior cornu with a well developed hippocampus minor. ' In many of these creatures, such as the Saimiri ( Cltrysothrix), the cerebral lobes overlap and extend much further behind the c.e~ebell~m, in proportion, than they do in man (Fig. 17)and It IS qui~e certain that, in all, the cerebellum is completely covered behind,- by well developed posterior lobes. The fact can be verified by every one who possesses the skull of any old or new world monkey. For, inasmuch as the brain in all mammals completely fills the cranial cavity, it is ob\'"ious that a cast of the i~terior of the skuU will reproduce the general form of the brain, at any rate with such minute and, for the present purpose, utterly unimportant differences as may result from the absence of the enveloping membranes of the brain in the dry skull. But if such a cast be made in plaster and compared with a similar cast of the interior of a h~man sk 11 it will b. e obvious that the cast of the cerebral chamb er, ure -' presenting the cerebrum of the ape, as completely covers over and overlaps the ca~t of the. cerebellar chamber, representing the cerebellum, as It does In the man (FI'g 21) A 1 • · c • • care ess ?bserver, forgetting that a soft structure like the .brain loses Its proper shape the moment it is taken out of the skull may indeed mistake the uncovered condition of the cere~ bell~m of an extracted and distorted brain for the natural relations of the parts; but his error must become patent even. to himself if he try to replace the brain within the ~ranial chamber. To suppose that the cerebellum of an ape Is naturally uncovered behind is a miscomprehension comparable only to that of one who should imagine that a man's H |