Structure of the Nervous System : Part 4

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 20 of 28)

Arrangement of the Neurons of the Brain and Cord. - The cell-bodies in the brain and spinal cord are collected into groups, and their fibers extend from these groups to places that may be near or remote. Guided by the white and gray colors of the nervous tissue, and also by the structures revealed by the microscope, physiologists have made out three general schemes in the grouping of cell-bodies, as follows:

1. That of surface distribution, the cell-bodies forming a thin but continuous layer over a given surface. This is the plan in the cerebrum and cerebellum, and here are found devices for increasing the surface: the cerebrum having convolutions, the cerebellum transverse ridges.

2. That of collections of cell-bodies into rounded masses. Such masses are found in the bulb, the pons, the midbrain, and the base of the cerebrum.

3. That of arrangement in a continuous column. This is the plan in the spinal cord. It matters not at what place the spinal cord be cut, a central area of gray matter, resembling in form the capital letter H, is always found.

The fibers connecting with the cell-bodies in the brain and spinal cord are gathered into bundles or tracts, and these pass through different parts somewhat as follows:

1. In the cerebrum they extend in three general directions, forming three classes of fibers. The first connect different localities in the same hemisphere, and are known as association fibers. The second make connection between the two hemispheres, and form the corpus callosum. These are known as commissural fibers. The third connect the cerebrum with the parts of the nervous system below, and are called projection fibers.

2. In the cerebellum both association and commissural fibers are found. Bands of fibers, passing upward toward the cerebrum and downward toward the cord, connect this part of the brain with other parts of the nervous system.

3. In the midbrain, bulb, and spinal cord fibers are found: first, that connect these parts with the cerebrum and cerebellum above; second, that pass into and become a part of the nerves of the body; and third, that connect the opposite sides of these parts together.

The Peripheral Division. - The peripheral division of the nervous system includes all the nervous structures found outside of the brain and spinal cord. These consist of the cranial, spinal, and sympathetic nerves, and of various small ganglia, all of which are closely connected with the central system.

Spinal Nerves and Dorsal-root Ganglia. - The spinal nerves comprise a group of thirty-one pairs, which connect the spinal cord with different parts of the trunk, with the upper, and with the lower extremities. Each nerve joins the cord by two roots, these being named from their positions the ventral, or anterior, root and the dorsal, or posterior, root. The two roots blend together within the spinal cavity to form a single nerve trunk, which passes out between the vertebra. On the dorsal root of each spinal nerve is a small ganglion which is named, from its position, the dorsal-root ganglion.

Double Nature of Spinal Nerves. - Charles Bell, in 1811, made the remarkable discovery that each spinal nerve is double in function. He found the portion connecting with the cord by the dorsal root to be concerned in the production of feeling and the portion connecting by the ventral root to be concerned in the production of motion. In keeping with these functions, the two divisions of the nerve are made up of different kinds of fibers, as follows:

1. The dorsal-root divisions, of the fibers of di-axonic neurons, the cell-bodies of which form the dorsal-root ganglia.

2. The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord.

The first convey impulses to the cord and are called afferent neurons; the second convey impulses from the cord and are known as efferent neurons. therefore, by forming a part of the nerve pathways between the skin and the brain, the dorsal divisions of these nerves aid in the production of feeling; and by completing pathways to the muscles, the ventral divisions aid in the production of motion.

The Cranial Nerves. - From the under front surface of the brain, twelve pairs of nerves emerge and pass to the head, neck, and upper portions of the trunk. These, the cranial nerves, have names suggestive of their function or distribution and, in addition, are given numbers which indicate the order in which they leave the brain. Unlike the spinal nerves, the cranial nerves present great variety among themselves, scarcely any two of them being alike in function or in their connection with different parts of the body. Several of them have to do with the special senses, and are for this reason very important. They connect the brain with the different parts of the head, neck, and trunk, as follows:

1. The first pair (olfactory nerves; nerves of smell; afferent) connect with the mucous membrane of the nostrils.

2. The second pair (optic nerves; nerves of sight; afferent) connect with the retina of the eyes.

3. The third, fourth, and sixth pairs connect with the internal and external muscles of the eyeballs.

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