Physiology and Hygiene for Secondary Schools

by Francis M. Walters

The Midbrain is a short, rounded, and compact body that lies immediately beneath the cerebrum, and connects[pg 290] 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 (Fig. 132).

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[pg 291] 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.

Fig. 132—Human brain viewed from below. C. Cerebrum. Cb. Cerebellum. M. Midbrain. P. Pons. B. Bulb. I-XII. Cranial nerves.

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.

[pg 292]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.[100] 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 (Fig. 133). 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.