property might
be spoken of as conductility.

When such an impression is thus conducted simply along a nerve fibre,
and thence to a muscle, it induces or excites, as we have seen, the
contraction of that muscle, and so exercises what is called a _motor_
function.

The nerve cells appear to possess, beyond the simple excitability to
general stimuli, conductility, and the peculiar receptivity which is
essential to sensation, a special or more exalted kind of excitability
which is called into play under mental or psychical stimuli by the
changes produced in the gray matter[5] in the formation of ideas,
emotions, and the will.[6]

Now if two sympathetic nerve systems operated upon by psychical stimuli
be directed to one and the same point, it is by no means difficult to
understand how the brains belonging to those systems may be brought into
telegraphic communication by means of the nerve fibres, the product of
the two minds evolved, and the resultant idea, by means of a simple
mechanical contrivance operated upon by the motor function already
explained, be transmitted to paper by the process of writing so familiar
to both. The action of the psychical stimuli on the nerve fibre, and its
transmission thence to the muscles resulting in the movement of the
board, is so subtle that we ourselves are not aware of its operation
except through the results produced.

It has just been said that two minds may be brought into telegraphic
communication by means of nerve fibres. Let us see how far the
expression is justified by facts. There are few of us who have not
experienced the truth of Solomon's saying that "if two persons lie
together, they have _heat_; but how can one be warm alone?" Even the
close proximity of two persons affects their respective temperatures,
and heat and motion we know to be correlative. It has been shown by the
physicist that mechanical force producing motion is correlative with and
convertible into heat, heat into chemical force, chemical force into
electrical force, and electrical force into magnetic force. Moreover,
that each of these is correlative and convertible into the other, all
being thus interchangeable.

"Now it is not to be supposed that the force acting in a nerve is
identical with electrical force, nor yet a peculiar kind of electricity,
nor even physically induced by it, as magnetism may be, but that in the
special action of the living nerve a force is generated peculiar to that
tissue