[pg 288]The Central Division.—This division of the nervous system lies within the cranial and spinal cavities, and consists of the brain and the spinal cord. The brain occupying the cranial cavity and the spinal cord in the spinal cavity connect with each other through the large opening at the base of the skull to form one continuous structure. The brain and cord are the most complicated portions of the nervous system, and the ones most difficult to understand.
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[pg 289] 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—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.