to the most singular
and, in some instances, even fatal results. So marvellously delicate is
this portion of our organization, that we are not always conscious of
this reaction, and as the reaction is conveyed from the nerve centres to
the muscular tissue, we actually find ourselves uttering words or making
motions unconsciously. So sensitive is the brain through the influence
of this higher nature, so subtle its functions, that it is often
impressed by means indiscernible to the bodily eye or to the ordinary
senses--by means just as mysterious as the action of magnetic attraction
or the course of the electric wave.

Byron alludes to this exquisite susceptibility with no less of truth
than beauty:

And slight withal may be the things which bring
Back on the heart the weight which it would fling
Aside for ever; it may be a sound,
A tone of music, summer's eve or spring,
A flower, the wind, the ocean, which shall wound,
_Striking the electric chain wherewith we are darkly bound_.

And how or why we know not, _nor can trace_
_Home to its cloud this lightning of the wind_ ...

Having referred to the reaction of a mental sensation on the nervous
system, let us now examine the course by which the reaction proceeds.

We are told by physiologists that stimuli applied to the nerves in
certain cases induce contraction or motion in the muscles by direct
conduction of a stimulus along a nerve, or by the conduction of a
stimulus to a nervous centre, whence it is reflected along another nerve
to the muscles. Not only mechanical and electrical, but _psychical_
stimuli "excite the nerves, whether these are ideational, emotional, or
volitional. They proceed from the brain, being themselves sometimes
induced by external causes, and sometimes originating primarily in the
great nervous centres from the _operations of the instinct, the memory,
the reason, or the will_."

When a stimulus of any kind, whether mechanical, chemical, electrical,
or vital, acts upon the living nervous substance, it produces an
impression on that nerve substance and excites within it some particular
change, and the property by which this takes place in the nerve
substance has been called its excitability or neurility. But the nerve
substance not only receives such an impression from a stimulus and is
excited to such a change, but it possesses the property of conducting
that impression in certain definite directions, and this