Let me illustrate this division by examples. Any ordinary act of the will, the voluntary movement of the arm tor instance, is effected by a mechanism to which the first impulse is given by a change in the mind; I will to move my arm; this mental change affects the nerves of the arm, which excite certain muscles to act.
That the nerves are the channels for conveying this influence of the will is proved, beyond all doubt, by experiment and disease. If their continuity with the brain be injured, the power is necessarily lost, although the will itself con tinue unimpaired.
We feel through the instrumentality of the nerves. In writing, I am conscious, through the sensibility of rny fingers, that 1 hold a pen in my hand. Were that sensibility destroyed, although the power of holding the pen re mained, I should lose the consciousness of its presence between my fingers, and they would cease to grasp it. This sensibility is due to the communication of the nerves with the brain, for any solution of continuity destroys it ; nor can any part be said to be sensible or to possess sensibility which does not communicate with the brain through the nerves. And various parts differ as regards the degree of sensibility which they enjoy, according to the number of nerves distributed to them, and perhaps also according to the manner in which the nerves are connected with them. A touch on the skin covering the olecranon is scarcely felt, whilst the finest point impinging with the slightest force on the skin of the tip of the finger is instantly perceived.
Sensations differ in kind as well as in degree. The power by which we are made sensible of contact, or by which, under the influence of undue stimulation, we become conscious of pain, is called common sensibility. Doubtle.s nearly all textures possess this to a certain degree. Tendon and cartilage enjoy a very much less amount of it than skin arid muscle. But we can also appreciate the influence of light, of sound, of ocloi, of flavour, and we are enabled to do this by means of particular nerves. 'This power is that of special sensibility. The nerves, which convey these iinpressions of special sensation to the mind, are incapable of responding in any other way, even to a me chanical stimulus. NVhen the retina is stimu
lated with the point of a needle, the sensation of a flash of light is produced : when the audi tory nerve is excited by a mechanical impulse upon the tympanum, sound is heard.
The mechanism (so to speak) of sensations of whatever kind is exactly in the reverse di rection to that of voluntary motions. In the latter the change begins in the mind and ends in the body ; in the former the impression is made first on the body and is conveyed to the mind. In both cases mental change, whether active or passive, is necessarily associated with the nervous act.
There are other actions in which the mind is concerned, althongh the wi// does not take any share in them. These are, such as rnay be produced under the influence of those sudden, momentary, and involuntary mental changes, which are called emotions, and which may be excited, either by an intpression conveyed to the mind from some external cause, through the senses, or by some change idthe mind itself, arising in the train of its thoughts. Who has not felt the thrill which pervades every part of the frame, in listening to some harrowing tale of woe? or, when the imagination, in its mu sings, conjures up before the mental vision some fearful scene of lamentation and wretched ness ? How keenly do the emotions of joy and sorrow, anger and pity, cause themselves to be felt at every point of the system. The blush of shame, the pale curling hp of anger, " theeye in a fine frenzy rolling," are al! examples of the emotions of the mind influencing bodily actions. The changes which the countenance undergoes in accordance with varying states of the mind result front the same cause. The will may acquire die power of controlling to a great extent this influence of einotion upon the ex pression of the features; but to attain this faculty in g,reat perfection requires great strength of will and frequency of exercise. Few men ever acquired such controul over the play of the features, and such power of resisting the influence of emotion as well as of imitating that influence, as Garrick and Talleyrand.