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