THE CEREBRO-SPINAL SYSTEM.
The Nervous Tissue.
The nervous system consists of the cerebrum, pons varolii, cerebellum, medulla oblongata, the spinal cord with its nerves and the sympathetic ganglia, etc.
The cerebrum or brain proper constitutes the highest and much the largest portion of the encephalon. The cerebrum consists of two halves, that are connected with each other by the corpus callosum, and with the peduncular masses of the cruri cerebri, the processus a cerebello ad cerebrum; the series of eminences, or cerebral centers or ganglia, concealed from view, named corpora quadrigemina, optic thalamus and corpora striata, etc.
The cerebral hemispheres are by far the most bulky part of the cerebrum. Various commissural structures unite the two hemispheres, including the corpus callosum and fornix; and some smaller structures, viz., the pineal gland, the petuitary bodies, and the olfactory bulb.
The cerebral hemispheres together form an ovoid mass, in contact with the vault of the cranium, and with its smaller end forward, its greatest width being opposite to the parietal eminences. They are separated in the greater part of their extent by the great longitudinal fissure.
The surface of the hemisphere is composed of gray matter, and is molded into numerous smooth tortuous eminences, named convolutions, or gyri, which are marked off from one another by deep furrows, called sulci.
The cerebrum is divided into lobes for convenience of study, five in number, called frontal, parietal, occipital, temporal, sphenoidal, and central.
The internal structure of the cerebrum is composed of white matter. It consists of tubular fibers varying in size in various parts, but in general still smaller than those in the cord, their average diameter being 1⁄10000 of an inch. The fibers of white substance present no division. They are arranged in bundles, separated by a network of delicate connective tissue, consisting of cells, etc. The cells are of various forms and sizes—spheroidal, angular, fusiform, etc. The fibers radiate from the white center of each convolution in all directions into the gray cortex, having a course for the most part perpendicular to the free surface. In passing through the gray substance they are arranged in bundles about 1⁄1500 of an inch in diameter, thus separating some of the nerve cells, etc.