Physiology and Hygiene for Secondary Schools

by Francis M. Walters

Many of the axons in the brain and spinal cord have no primitive sheath. Axons without the medullary sheath are found in the sympathetic nerves. These are known as non-medullated axons and they have a gray instead of a white color.

The difference in weight between the brain of man and that of woman is due mainly to the fact that man's body is, as a rule, considerably larger than that of woman's.

The nervous tissues present, at different places, two colors—one white, and the other a light gray. Great significance was formerly attached to these colors, because it was supposed that they represented two essentially different kinds of nervous matter. It is now known that the protoplasm in all parts of the neuron proper—cell-body, axis cylinder, and dendrites—has a grayish color, while the coverings of most of the fibers are white. Hence gray matter in any part of the nervous system indicates the presence of cell-bodies, and white matter the presence of nerve fibers.

In very early life the spinal cord entirely fills the spinal cavity, but as the body develops the cord grows less rapidly than the spinal column, and, as a consequence, separates at the lower end from the inclosing bony column.

Fibers passing between the spinal cord and the cerebrum cross to opposite sides—most of them at the bulb, but many within the cord—so that the right side of the cerebrum is connected with the left side of the body, and vice versa. This accounts for the observed fact that disease or accidental injury of one side of the cerebrum causes loss of motion or of feeling in the opposite side of the body.

In general, afferent neurons or fibers are those that convey impulses toward the central nervous system (brain and cord), while efferent neurons or fibers are those that convey impulses from the central system.

At different times the nervous impulse has been regarded as a current of electricity; as a progressive chemical change, likened to that in a burning fuse; as a mechanical vibration, such as may be passed over a stretched rope; and as a molecular disturbance accompanied by an electrical discharge. The velocity of the nervous impulse, which is only about one hundred feet per second, proves that it is not a current of electricity. It takes place with little or no exhaustion of the cell protoplasm and consequently is not due to chemical action. And the loose, relaxed condition of the nerves prevents their transmission of physical vibrations, like those on a stretched rope. The view that the impulse is a progressive molecular disturbance, accompanied by an electrical discharge, has much evidence in its favor, but it has only recently been proposed and is likely to be modified upon fuller investigation.

The surface of the body includes the linings of the air passages, food canal, and certain cavities, as well as the external covering or skin.

Derived from the Latin re, back, and flectere, to turn or bend.

A frog from which the brain has been removed is suspended with its feet downward and free to move. If a toe is pinched, the foot is drawn away, and if dilute acid, or a strong solution of salt, is placed on the tender skin, the feet are moved as if to take away the irritating substance. This of course shows that reflex action can take place independently of the brain.