al mammals, the cerebral hemispheres leave the
proper upper and posterior face of the cerebellum completely visible,
when the brain is viewed from above; but, in the higher forms, the
hinder part of each hemisphere, separated only by the tentorium (p.
281) from the anterior face of the cerebellum, inclines backwards and
downwards, and grows out, as the so-called "posterior lobe," so as at
length to overlap and hide the cerebellum. In all Mammals, each cerebral
hemisphere contains a cavity which is termed the 'ventricle,' and as
this ventricle is prolonged, on the one hand, forwards, and on the other
downwards, into the substance of the hemisphere, it is said to have two
horns or 'cornua, an 'anterior cornu,' and a 'descending cornu.' When
the posterior lobe is well developed, a third prolongation 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 substance 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 meanderings, 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 impossibility of
erecting any cerebral barrier between man and the apes, Nature has
provided us, in the latter animals, with an