ant 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 human skull, it will be obvious that the cast of the cerebral chamber,
representing the cerebrum of the ape, as completely covers over
and overlaps the cast of the cerebellar chamber, representing the
cerebellum, as it does in the man (Fig. 20). A careless observer,
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 cerebellum 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 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 oblique