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.

The third property of nerve-cells which is important in study is
_modifiability_. That is, impressions made upon the nerve-cells are
retained. Most living tissue is modifiable to some extent. The features
of the face are modifiable, and if one habitually assumes a peevish
expression, it becomes, after a time, permanently fixed. The nervous
system, however, possesses the power of modifiability to a marked
degree, even a single impression sufficing to make striking
modification. This is very important in study, being the basis for the
retentive powers of the mind.

Having examined the action of the nervous system in its simplicity, we
have now to examine the ways in which the parts of the nervous system
are combined. We shall be helped if we keep to the conception of it as
an aggregation of systems or groups of pathways. Some of these we shall
attempt to trace out. Beginning with those at the outermost parts of
the body, we find them located in the sense-organs, not only within
the traditional five, but also within the muscles, tendons, joints,
and internal organs of the body such as the heart, and digestive
organs. In all these places we find ends of neurones which converge at
the spinal cord and travel to the brain. They are called sensory
neurones and their function is to carry messages inward to the brain.
Thus, the brain represents, in great part, a central receiving station