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 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]