Structure of the Nervous System : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 17 of 28)

Coordination and Adjustment. - If we consider for a moment the movements of the body, we cannot fail to note the cooperation of organs, one with another. In the simple act of whittling a stick one hand holds the stick and the other the knife, while the movements of each hand are such as to aid in the whittling process. Examples of cooperation are also found in the taking of food, in walking, and in the performance of different kinds of work.

Not only is cooperation found among the external organs, but our study of the vital processes has shown that the principle of cooperation is carried out by the internal organs as well. The fact that all the activities of the body are directed toward a common purpose makes the cooperation of its parts a necessity. The term "coordination" is employed to express this cooperation, or working together, of the different parts of the body.

A further study of the movements of the body shows that many of them have particulary reference to things outside of it. In going about one naturally avoids obstructions, and if anything is in the way he walks around or steps over it. Somewhat as a delicate instrument (the microscope for example) is altered or adjusted, in order to adapt it to its work, the parts of the body, and the body as a whole, have to be adjusted to their surroundings. This is seen in the attitude assumed in sitting and in standing, in the position of the hands for different kinds of work, in the variations of the circulation of the blood in the skin, and in the movements for protecting the body.

Work of the Nervous System. - How are the different activities of the body controlled and coordinated? How is the body adjusted to its surroundings? The answer is found in the study of the nervous system. Briefly speaking, the nervous system controls, coordinates, and adjusts the different parts of the body by fulfilling two conditions:

1. It provides a complete system of connections throughout the body, thereby bringing all parts into communication.

2. It supplies a means of controlling action (the so-called impulse) which it passes along the nervous connections from one part of the body to another.

The present chapter deals with the first of these conditions; the chapter following, with the second.

The Nerve Skeleton. - If all the other tissues are removed, leaving only the nervous tissue, a complete skeleton outline of the body still remains. This nerve skeleton, as it has been called, has the general form of the framework of bones, but differs from it greatly in the fineness of its structures and the extent to which it represents every portion of the body. An examination of a nerve skeleton, or a diagram of one, shows the main structures of the nervous system and their connection with the different parts of the body.

Corresponding to the skull and the spinal column is a central nervous axis, made up of two parts, the brain and the spinal cord. From this central axis white, cord-like bodies emerge and pass to different parts of the body. These are called nerve trunks, and the smaller branches into which they divide are called nerves. The nerves also undergo division until they terminate as fine thread-like structures in all parts of the body. The distribution of nerve terminations, however, is not uniform, as might be supposed, but the skin and important organs like the heart, stomach, and muscles are the more abundantly supplied. On many of the nerves are small rounded masses, called ganglia, and from many of these small nerves also emerge. At certain places the nerves and ganglia are so numerous as to form a kind of network, known as a plexus.

It is through these structures - brain and spinal cord, nerve trunks and nerves, ganglia and nerve terminations - that connections are established between all parts of the body, but more especially between the surface of the body and the organs within.

The Neurons, or Nerve Cells. - While a hasty examination of the nerve skeleton is sufficient to show the connection of the nervous system with all parts of the body, no amount of study of its gross structures reveals the nature of its connections or suggests its method of operation. Insight into the real nature of the nervous system is obtained only through a study of its minute structural elements. These, instead of being called cells, as in the case of the other tissues, are called neurons. The use of this term, instead of the simpler one of nerve cell, is the result of recent advances in our knowledge of the nervous system.

The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances. Though the neurons in certain parts of the body differ greatly in form and size from those in other parts of the body, most of them may be included in one or the other of two classes, known as mon-axonic neurons and di-axonic neurons.

Mon-axonic Neurons. - Neurons of this class consist of three distinct parts, known as the cell-body, the dendrites, and the axon.

The cell-body has in itself the form of a complete cell and was at one time so described. It consists of a rounded mass of protoplasm, containing a well-defined nucleus. The protoplasm is similar to that of other cells, but is characterized by the presence of many small granules and has a slightly grayish color.

The dendrites are short extensions from the cell-body. They branch somewhat as the roots of a tree and form in many instances a complex network of tiny rootlets. Their protoplasm, like that of the cell-body, is more or less granular. The dendrites increase greatly the surface of the cell-body, to which they are related in function.

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