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tion of the ventricular cavity extends into it, and is called the 
* 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 sub- 
stance of the brain ; and the smaller species of all orders tend 
to a similar 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 complicated in their mean- 
derings, until, in the Elephant, the Porpoise, the higher Apes, 
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 cornu— 
which is, as it were, arched over the roof of the sulcus. It 
is as if the groove had been formed by indenting the floor of 
the posterior horn from without with a blunt instrument, so 
that the floor should rise as a convex 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 impossi- 
bility 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 
