error must become patent even to himself if he try to replace the brain
within the cranial 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 lungs always occupy but a
small portion of the thoracic cavity--because they do so when the chest
is opened, and their elasticity is no longer neutralized by the
pressure of the air.

And the error is the less excusable, as it must become apparent to every
one who examines a section of the skull of any ape above a Lemur,
without taking the trouble to make a cast of it. For there is a very
marked groove in every such skull, as in the human skull--which
indicates the line of attachment of what is termed the 'tentorium'--a
sort of parchment-like shelf, or partition, which, in the recent state,
is interposed between the cerebrum and cerebellum, and prevents the
former from pressing upon the latter. (See Fig. 16.)

This groove, therefore, indicates the line of separation between that
part of the cranial cavity which contains the cerebrum, and that which
contains the cerebellum; and as the brain exactly fills the cavity of
the skull, it is obvious that the relations of these two parts of the
cranial cavity at once informs us of the relations of their contents.
Now in man, in all the old-world, and in all the new-world Simiae, with
one exception, when the face is directed forwards, this line of
attachment of the tentorium, or impression for the lateral sinus, as it
is technically called, is nearly horizontal, and the cerebral chamber
invariably overlaps or projects behind the cerebellar chamber. In the
Howler Monkey or 'Mycetes' (see Fig. 16), the line passes obliquely
upwards and backwards, and the cerebral overlap is almost nil; while in
the Lemurs, as in the lower mammals, the line is much more inclined in