The nervous system, in its properties and mode of action, is distinct from all the other systems and organs, and it shares with no other organ or tissue the power to do its special work.  It is the medium through which all impressions are received.  It connects all the parts of the body into an organism in which each acts in harmony with every other part for the good of the whole.  It animates and governs all movements, voluntary or involuntary,—­secretion, excretion, nutrition; in fact all the processes of organic life are subject to its regulating power.  The different organs of the body are united by a common sympathy which regulates their action:  this harmonious result is secured by means of the nervous system.

This system, in certain of its parts, receives impressions, and generates a force peculiar to itself.  We shall learn that there can be no physical communication between or cooerdination of the various parts of organs, or harmonious acts for a desire result, without the nerves.  General impressions, as in ordinary sensation, or special impressions, as in sight, smell, taste, or hearing,—­every instinct, every act of the will, and every thought are possible only through the action of the nerve centers.

261.  Nerve Cells.  However complicated the structure of nerve tissue in man seems to be, it is found to consist of only two different elements, nerve cells and nerve fibers.  These are associated and combined in many ways.  They are arranged in distinct masses called nerve centers, or in the form of cords known as nerves.  The former are made up of nerve fibers; the latter of both cells and fibers.

[Illustration:  Fig. 110.  Nerve Cells from the Spinal Cord.]

Nerve cells, which may be regarded as the central organs of the nerve fibers, consist of masses of cell protoplasm, with a large nucleus and nucleolus.  They bear a general resemblance to other cells, but vary much in size and shape.  Nerve cells grow, become active, and die, as do other cells.  A number of processes branch off from them, some cells giving one or two, others many.  The various kinds of nerve cells differ much in the shape and number of processes.  One of the processes is a strand which becomes continuous with the axis cylinder of the nerve fibers; that is, the axis cylinders of all nerve fibers are joined in one place or another with at least one cell.

Each part of this system has its own characteristic cell.  Thus we have in the spinal cord the large, irregular cells with many processes, and in the brain proper the three-sided cells with a process jutting out from each corner.  So characteristic are these forms of cells, that any particular part of nerve structure may be identified by the kind of cells seen under the microscope.  Nerve cells and nerve fibers are often arranged in groups, the various cells of the groups communicating with one another.  This clustered arrangement is called a nerve center.