Structure of the Nervous System : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 18 of 28)

The axon, or nerve fiber, is a long, slender extension from the cell-body, which connects with some organ or tissue. It was at one time described as a distinct nervous element, but later study has shown it to be an outgrowth from the cell-body. The mon-axonic neurons are so called from their having but a single axon.

Di-axonic Neurons. - Neurons belonging to this class have each a well-defined cell-body and two axons, but no parts just like the dendrites of mon-axonic neurons. The cell-body is smooth and rounded, and its axons extend from it in opposite directions.

Structure of the Axon. - The axon, or nerve fiber, has practically the same structure in both classes of neurons, being composed in most cases of three distinct parts. In the center, and running the entire length of the axon, is a thread-like body, called the axis cylinder. The axis cylinder is present in all axons and is the part essential to their work. It may be considered as an extension of the protoplasm from the cell-body. Surrounding the axis cylinder is a thick, whitish-looking layer, known as the medullar sheath, and around this is a thin covering, called the primitive sheath, or neurilemma. The medullar sheath and the primitive sheath are not, strictly speaking, parts of the nerve cell, but appear to be growths that have formed around it. Certain of the axons have no primitive sheath and others are without a medullar sheath.

Form and Length of Axons. - Where the axons terminate they usually separate into a number of small divisions, thereby increasing the number of their connections. Certain axons are also observed to give off branches before the place of termination is reached. These collateral branches, by distributing themselves in a manner similar to the main fiber, greatly extend the influence of a single neuron.

In the matter of length, great variation is found among the axons in different parts of the body. In certain parts of the brain, for example, are fibers not more than one one-hundredth of an inch in length, while the axons that pass all the way from the spinal cord to the toes have a length of more than three feet. Between these extremes practically all variations in length are found.

Arrangements of the Neurons. - Nowhere in the body do the neurons exist singly, but they are everywhere connected with each other to form the different structures observed in the nerve skeleton. Two general plans of connection are to be observed, known as the anatomical and the physiological, or, more simply speaking, as the "side-by-side" and "end-to-end" plans. The side-by-side plan is seen in that disposition of the neurons which enables them to form the nerves and the ganglia, as well as the brain and spinal cord. The end-to-end connections are necessary to the work which the neurons do.

Side-by-side Connections. - On separating the ganglia and nerves into their finest divisions, it is found that the nerves consist of axons, while the ganglia are made up mainly of cell-bodies and dendrites. The axons lie side by side in the nerve, being surrounded by the same protective coverings, while the cell-bodies form a rounded mass or cluster, which is the ganglion. But the axons, in order to connect with the cell-bodies, must terminate within the ganglion, so that they too form a part of it. To some extent, also, axons pass through ganglia with which they make no connection. The neurons in the brain and spinal cord also lie side by side, but their arrangement is more complex than that in the nerves and ganglia.

The side-by-side arrangement of the neurons shows clearly the structure of the ganglia and nerves. The nerve is seen to be a bundle of axons, or nerve fibers, held together by connective tissue, while the ganglion is little more than a cluster of cell-bodies. Their connection is necessarily very close, for the same group of neurons will form, with their axons, the nerve, and, with their cell-bodies, the ganglion.

End-to-end Connections. - These consist of loose end-to-end unions of the fiber branches of certain neurons with the dendrites of other neurons. The purpose of such connections is to provide the means of communication between different parts of the body. There appears to be no actual uniting of the fiber branches with the dendrites, but they come into relations sufficiently close to establish conduction pathways, and these extend throughout the body. They connect all parts of the body with the brain and spinal cord, while connections within the brain and cord bring the parts into communication with each other.

Nature of the Nervous System. - The nervous system represents the sum total of the neurons in the body. In some respects it may be compared to the modern telephone system. The neurons, like the electric wires, connect different places with a central station (the brain and spinal cord), and through the central station connections are established between the different places in the system. As the separate wires are massed together to form cables, the neurons are massed to form the gross structures of the nervous system. The nervous system, however, is so radically different from anything found outside of the animal body that no comparison can give an adequate idea of it. We now pass to a study of the gross structures observed in the nerve skeleton.

Divisions of the Nervous System. - While all of the nervous structures are very closely blended, forming one complete system for the entire body, this system presents different divisions which may, for convenience, be studied separately. As physiologists have become better acquainted with the human nervous system, different schemes of classification have been proposed. The following outline, based upon the location of the different parts, presents perhaps the simplest view of the entire group of nervous structures:

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