arently new
structure is found between the cerebral hemispheres, connecting them
together, as what is called the 'great commissure' or 'corpus callosum.'
The subject requires careful re-investigation, but if the currently
received statements are correct, the appearance of the 'corpus callosum'
in the placental mammals is the greatest and most sudden modification
exhibited by the brain in the whole series of vertebrated animals--it is
the greatest leap anywhere made by Nature in her brain work. For the
two halves of the brain being once thus knit together, the progress of
cerebral complexity is traceable through a complete series of steps from
the lowest Rodent, or Insectivore, to Man; and that complexity consists,
chiefly, in the disproportionate development of the cerebral hemispheres
and of the cerebellum, but especially of the former, in respect to the
other parts of the brain.

In the lower placental 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 per