According to present knowledge, the action of the nervous system is best conceived as a form of chemical change that spreads among the nerve-cells. We call this commotion the nervous current. It is very rapid, moving faster than one hundred feet a second, and runs along the cells in much the same way as a "spark runs along a train of gunpowder." It is important to note that neurones never act singly; they always act in groups, the nervous current passing from neurone to neurone. It is thought that the most important changes in the nervous system do not occur within the individual neurones, but at the points where they join with each other. This point of connection is called the synapse and although we do not understand its exact nature, it may well be pictured as a valve that governs the passage of the nervous current from neurone to neurone. At time of birth, most of the valves are closed. Only a few are open, mainly those connected with the vegetative processes such as breathing and digestion. But as the individual is played upon by the objects of the environment, the valves open to the passage of the nervous current. With increased use they become more and more permeable, and thus learning is the process of making easier the passage of the nervous current from one neurone to another. We shall secure further light upon the action of the nervous system if we examine some of the properties belonging to nerve-cells. The first one is _impressibility_. Nerve-cells are very sensitive to impressions from the outside. If you have ever had the dentist touch an exposed nerve, you know how extreme this sensitivity is. Naturally such a property is very important in education, for had we not the power to receive impressions from the outside world we should not be able to acquire knowledge. We should not even be able to perceive danger and remove ourselves from harm. "If we compare a man's body to a building, calling the steel frame-work his skeleton and the furnace and power station his digestive organs and lungs, the nervous system would include, with other things, the thermometers, heat regulators, electric buttons, door-bells, valve-openers,--the parts of the building, in short, which are specifically designed to respond to influences of the environment." The second property of nerve-cells which is important in study is _conductivity_. As soon as a neurone is stimulated at one end, it communicates its excitement, by means of the nervous current, to the next neurone or to neighboring neurones. Just as an electric current might pass along one wire, thence to another, and along it to a third, so the nervous current passes from neurone to neurone. As might be expected, the two functions of impressibility and conductivity are aided by such an arrangement of the nerve-cells that the nervous current may pass over definitely laid pathways. These systems of pathways will be described in a later paragraph.