vous system is made up of millions of these little cells
packed together in various combinations and distributed throughout the
body. Some of the neurones are as long as three feet; others measure
but a fraction of an inch in length.

We do not know exactly how the mind, that part of us which feels,
reasons and wills, is connected with this mass of cells called the
nervous system. We do know, however, that every time anything occurs in
the mind, there is a change in some part of the nervous system.
Applying this fact to study, it is obvious that when you are performing
any of the operations of study, memorizing foreign vocabularies, making
arithmetical calculations, reasoning out problems in geometry, you are
making changes in your nervous system. The question before us, then,
is, What is the nature of these changes?

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 sensitivi