Internal Construction of the Nerves and Nerve Centers
We shall understand nerve action better if we know something of the way in which the nervous system is built. A nerve is not to be thought of as a unit, nor are the brain and cord to be thought of as mere masses of some peculiar substance.
Fig. 4.--A motor nerve cell from the spinal cord, highly magnified. (Figure text: dendrites, cell body, axon, termination of axon in muscle)
A nerve is a bundle of many slender insulated threads, just as a telephone cable, running along the street, [{32}] is a bundle of many separate wires which are the real units of telephonic communication. A nerve center, like the switchboard in a telephone central, consists of many parts and connections.
The whole nervous system is essentially composed of neurones. A neurone is a nerve cell with its branches. Most nerve cells have two kinds of branches, called the axon and the dendrites.
The nerve cell is a microscopic speck of living matter. Its dendrites are short tree-like branches, while its axon is often several inches or even feet in length. The axon is the "slender thread", just spoken of as analogous to the single telephone wire. A nerve is composed of axons. [Footnote: The axon is always protected or insulated by a sheath, and axon and sheath, taken together, are often called a "nerve fiber".] The "white matter" of the brain and cord is composed of axons. Axons afford the means of communication between the nerve centers and the muscles and sense organs, and between one nerve center and another.
The axons which make up the motor nerves are branches of nerve cells situated in the cord and brain stem; they extend from the reflex center for any muscle out to and into that muscle and make very close connection with the muscle substance. A nerve current, starting from the nerve cells in the reflex center, runs rapidly along the axons to the muscle and arouses it to activity.
The axons which make up the optic nerve, or nerve of sight, are branches of nerve cells in the eye, and extend into the brain stem. Light striking the eye starts nerve currents, which run along these axons into the brain stem. Similarly, the axons of the nerve of smell are branches of cells in the nose.
The remainder of the sensory axons are branches of nerve cells that lie in little bunches close alongside the cord or [{33}] brain stem. These cells have no dendrites, but their axon, dividing, reaches in one direction out to a sense organ and in the other direction into the cord or brain stem, and thus connects the sense organ with its "lower center".
Fig. 5.--Sensory and motor axons, and their nerve cells. The arrows indicate the direction of conduction. (Figure text: eye, brain stem, skin, cord, muscle)
Where an axon terminates, it broadens out into a thin plate, or breaks up into a tuft of very fine branches ( the "end-brush"), and by this means makes close contact with the muscle, the sense organ, or the neurone with which it connects.