this sensory cell could make would depend on the number of its branches; but, as a matter of fact, most of the branches from the sensory cells make nerve junctions with connecting cells, and not with motor cells directly. These connecting cells in turn are branched and many of these branches lead again to connecting cells, so we see that the number of connections that a single sensory cell can make quickly becomes very large. The arrangement is shown in the accompanying diagram. Not only does one sensory cell have in this way the possibility of nerve connection with all the muscles, but the reverse is also true; namely, that every muscle has the possibility of being connected with every sense organ. This means that there must be a number of nerve junctions connecting with the cell body of each of the motor nerve cells. If we look back at the diagram of the motor nerve cell, we shall see that it has a great many tiny branches leading off it. These are numerous enough to enable the feathery tips of a great many sensory or connecting nerve cells to interweave with them, and so enable any motor cell to be acted upon from a great many different directions.
The central nervous system, which is the place where all these much-branched pathways are and where all the nerve junctions are located, is made up of two chief parts, the brain and the spinal cord. The brain is inside the skull and the spinal cord is an extension down the back. It lies in the tunnel made up of the arches of the bones of the vertebral column as described in Chapter VI. The cables which contain the axons of both sensory and motor nerve cells extend from the brain or from the spinal cord out to the different parts of the body where the sense organs and the muscles are located. They start as large nerve trunks which divide and subdivide as they get farther and farther away from the central nervous system. The large nerve trunks are arranged in pairs—those that spring from the brain are called cranial nerves; those that spring from the spinal cord are called spinal nerves. The cranial nerves, with the exception of one, lead only to points in the head or neck, and so are short; the spinal nerves on the other hand reach to the various parts of the trunk or down the arms and legs, so that some of these may be three feet long or more. An idea of the arrangement of the nervous system is given in the accompanying figure.
A good way to realize the actual working of the nervous system is to take a particular action and follow it through. Suppose a barefooted boy steps on a sharp thorn. The thorn arouses some of the sense organs in the sole of the foot. These in turn start a disturbance in the sensory nerve cells which pass up the leg to the lower end of the spinal cord in the small of the back. Within the spinal cord the sensory cells branch and the disturbance set up by the prick of the thorn spreads all over these branches to their tips. Some of the nerve junctions thus affected lead to muscles which will cause the foot to be jerked up; others communicate with connecting nerve cells which extend all the way up the spinal cord and into the brain and make the boy aware of the fact that he has stepped on the thorn; still others may make connection with muscles which would cause him to sit down and look at the bottom of his foot; still others may lead to the vocal muscles and the tear glands, causing the boy to cry. Of course these are not the only possible nerve connections; every muscle in the body might theoretically be aroused to action as the result of the stepping on the thorn. As a matter of fact, there is a condition in which this will happen. The drug strychnine has such an effect upon the nervous system that the stimulation of any sense organ actually does arouse all
