rain and cord are
the most complicated portions of the nervous system, and the ones most
difficult to understand.

[Fig. 130]

Fig. 130--*Diagram of divisions of brain.*

*The Brain.*--The brain, which is the largest mass of nervous tissue in the
body, weighs in the average sized man about 50 ounces, and in the average
sized woman about 44 ounces.(98) It may be roughly divided into three
parts, which are named from their positions (in lower animals) the
forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain
consists almost entirely of a single part, known as

*The Cerebrum.*--The cerebrum comprises about seven eighths of the entire
brain, and occupies all the front, middle, back, and upper portions of the
cranial cavity, spreading over and concealing, to a large extent, the
parts beneath. The surface layer of the cerebrum is called the _cortex_.
This is made up largely of cell-bodies, and has a grayish appearance.(99)
The cortex is greatly increased in area by the presence everywhere of
ridge-like _convolutions_, between which are deep but narrow depressions,
called _fissures_. The interior of the cerebrum consists mainly of nerve
fibers, or axons, which give it a whitish appearance. These fibers connect
with the cell-bodies in the cortex (Fig. 131).

The cerebrum is a double organ, consisting of two similar divisions,
called the _cerebral hemispheres_. These are separated by a deep groove,
extending from the front to the back of the brain, known as the _median
fissure_. The hemispheres, however, are closely connected by a great band
of underlying nerve fibers, called the _corpus callosum_.

[Fig. 131]

Fig. 131--*Microscope drawing* of a neuron from cerebral cortex. _a._ Short
segment of the axis cylinder with collateral branches.

At the base of the cerebrum three large masses of cell-bodies are to be
found. One of these, a double mass, occupies a central position between
the hemispheres, and is called the _optic thalami_. The other two occupy
front central positions at the base of either hemisphere, and are known as
the _corpora striata_, or the striate bodies.

*The Midbrain* is a short, rounded, and compact body that lies immediately
beneath the cerebrum, and connects it with the hindbrain. On account of
the great size of the cerebrum, the midbrain is entirely concealed from
view when the other p