ated fibers--that is of both "white" and
"gray" matter. In the medulla, the unmedullated neurones which comprise
the center of the cord are passing to the outside, and the medullated to
the inside, thus taking the positions they occupy in the cerebrum. Here
also the neurones are crossing, or changing sides, so that those which
pass up the right side of the cord finally connect with the left side of
the brain, and vice versa.

THE CEREBELLUM.--Lying just back of the medulla and at the rear part of
the base of the cerebrum is the cerebellum, or "little brain,"
approximately as large as the fist, and composed of a complex
arrangement of white and gray matter. Fibers from the spinal cord enter
this mass, and others emerge and pass on into the cerebrum, while its
two halves also are connected with each other by means of cross fibers.

[Illustration: FIG. 8.--View of the under side of the brain. B, basis of
the crura; P, pons; Mo, medulla oblongata; Ce, cerebellum; Sc, spinal
cord.]

THE CEREBRUM.--The cerebrum occupies all the upper part of the skull
from the front to the rear. It is divided symmetrically into two
hemispheres, the right and the left. These hemispheres are connected
with each other by a small bridge of fibers called the _corpus
callosum_. Each hemisphere is furrowed and ridged with convolutions, an
arrangement which allows greater surface for the distribution of the
gray cellular matter over it. Besides these irregularities of surface,
each hemisphere is marked also by two deep clefts or _fissures_--the
fissure of Rolando, extending from the middle upper part of the
hemisphere downward and forward, passing a little in front of the ear
and stopping on a level with the upper part of it; and the fissure of
Sylvius, beginning at the base of the brain somewhat in front of the
ear and extending upward and backward at an acute angle with the base
of the hemisphere.

[Illustration: FIG. 9.--Diagrammatic side view of brain, showing
cerebellum (CB) and medulla oblongata (MO). F' F'' F''' are placed on
the first, second, and third frontal convolutions, respectively; AF, on
the ascending frontal; AP, on the ascending parietal; M, on the
marginal; A, on the angular. T' T'' T''' are placed on the first,
second, and third temporal convolutions. R-R marks the fissure of
Rolando; S-S, the fissure of Sylvius; PO, the parieto-occipital
fissure.]

The surface of each hemisphere may be thought of as mapped out into four
lo