Structure of the Nervous System : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 19 of 28)

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.

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. It may be roughly divided into three parts, which are named from their positions (in lower animals) the forebrain, the midbrain, and the hindbrain. 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. 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.

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.

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 parts occupy their normal positions. However, if the cerebrum is pulled away from the hindbrain, it is brought into view somewhat as in Fig. 130.

The midbrain carries upon its back and upper surface four small rounded masses of cell-bodies, called the corpora quadrigemina. The upper two of these bodies are connected with the eyes; the lower two appear to have some connection with the organs of hearing. On the front and under surface, the midbrain separates slightly as if to form two pillars, which are called the crura cerebri, or cerebral peduncles. These contain the great bundles of nerve fibers that connect the cerebrum with the parts of the nervous system below.

The Hindbrain lies beneath the back portion of the cerebrum, and occupies the enlargement at the base of the skull. It forms about one eighth of the entire brain, and is composed of three parts - the cerebellum, the pons, and the bulb.

The Cerebellum is a flat and somewhat triangular structure with its upper surface fitting into the triangular under surface of the back of the cerebrum. It is divided into three lobes - a central lobe and two lateral lobes - and weighs about two and one half ounces. In its general form and appearance, as well as in the arrangement of its cell-bodies and axons, the cerebellum resembles the cerebrum. It differs from the cerebrum, however, in being more compact, and in having its surface covered with narrow, transverse ridges instead of the irregular and broader convolutions.

The Pons, or pons Varolii, named from its supposed resemblance to a bridge, is situated in front of the cerebellum, and is readily recognized as a circular expansion which extends forward from that body. It consists largely of bands of nerve fibers, between which are several small masses of cell-bodies. The fibers connect with different parts of the cerebellum and with parts above.

The Bulb, or medulla oblongata, is, properly speaking, an enlargement of the spinal cord within the cranial cavity. It is somewhat triangular in shape, and lies immediately below the cerebellum. It contains important clusters of cell-bodies, as well as the nerve fibers that pass from the spinal cord to the brain.

The Spinal Cord. - This division of the central nervous system is about seventeen inches in length and two thirds of an inch in diameter. It does not extend the entire length of the spinal cavity, as might be supposed, but terminates at the lower margin of the first lumbar vertebra. It connects at the upper end with the bulb, and terminates at the lower extremity in a number of large nerve roots, which are continuous with the nerves of the hips and legs. Two deep fissures, one in front and the other at the back, extend the entire length of the cord, and separate it into two similar divisions. These are connected, however, along their entire length by a central band consisting of both gray and white matter.

The arrangement of the neurons of the spinal cord is just the reverse of that in the cerebrum - the center being occupied by a double column of cell-bodies, which give it a grayish appearance, while the fibers occupy the outer portion of the cord, giving it a whitish appearance.

The spinal cord is not uniform in thickness, but tapers slightly, though not uniformly, from the upper toward the lower end. At the places where the nerves from the arms and legs enter the cord two enlargements are to be found, the upper being called the cervical and the lower the lumbar enlargement. These, on account of the difference in length between the cord and the spinal cavity, are above - the lower one considerably above - the places where the limbs which they supply join the trunk.

D.C. Heath and Co. - Publishers
Original copyright 1909