iption of which
will follow. The cerebral surface, above and in front of the corpus
callosum, is divided into two by a sulcus, the contour of which
closely resembles that of the upper margin of the corpus callosum.
This is the _calloso-marginal sulcus_, so called because it separates
the callosal gyrus, which lies between it and the corpus callosum,
from the marginal gyri nearer the margin of the brain. When the sulcus
reaches a point vertically above the hind end of the corpus callosum
it turns sharply upward and so forms the hinder limit of the marginal
gyri, the posterior inch or two of which is more or less distinctly
marked off to form the _paracentral lobule_, where the upper part of
the central fissure of Rolando turns over the margin of the brain. The
callosal gyrus, which is also called the gyrus fornicatus from its
arched appearance, is continued backward round the posterior end of
the corpus callosum, and so to the mesial surface of the temporal
lobe. Behind the upturned end of the calloso-marginal sulcus there is
a square area which is called the _precuneus_ or _quadrate lobe_; it
is bounded behind by the deeply cut internal parieto-occipital fissure
and this runs from the margin of the brain downward and forward to
join another fissure, the calcarine, at an acute angle, thus enclosing
a wedge-shaped piece of brain called the _cuneus_ between them. The
_calcarine_ fissure is fairly horizontal, and is joined about its
middle by the internal parieto-occipital, so that the part in front
of the junction is called the _pre-calcarine_, and that behind the
_post-calcarine_ fissure. The internal parieto-occipital and calcarine
are real fissures, because they cause an elevation in the interior of
the brain, known as the hippocampus minor. Just in front of the
anterior end of the calcarine fissure the callosal gyrus is
constricted to form the isthmus which connects it with the hippocampal
or uncinate gyrus. Below the calcarine fissure is a gyrus called the
_gyrus lingualis_, and this is bounded below by another true fissure,
the _collateral_, which runs parallel to the calcarine, but is
continued much farther forward into the temporal lobe and so forms the
lower boundary of the hippocampal gyrus. It will thus be seen that the
hippocampal gyrus is continuous posteriorly with the callosal gyrus
above by means of the isthmus, and with the gyrus lingualis