Physiology and Hygiene for Secondary Schools

by Francis M. Walters

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 (Fig. 125), 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. [pg 281]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.

Fig. 125—Diagram of nerve skeleton. The illustration shows the extent and general arrangement of the nervous tissue. A. Brain. B. Spinal cord. N. Nerve trunks and nerves. G. Ganglia.

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[pg 282] 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.[96]

Fig. 126—Diagram of a mon-axonic neuron (greatly enlarged except as to length). The central thread in the axon is the axis cylinder.

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.