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قراءة كتاب Mind and Motion and Monism

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Mind and Motion and Monism

Mind and Motion and Monism

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دار النشر: Project Gutenberg
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act of will in the brain, which serves as a stimulus to the contraction of the muscle, is accompanied by a vibratory movement in the grey matter of the brain; that this movement is going on at the rate of nine pulsations per second; and that the muscle is giving a separate or distinct contraction in response to every one of these nervous pulsations. That such is the true explanation of the rhythm in the muscle is proved by the fact that if, instead of contracting a muscle by an act of the will, it be contracted by means of a rapid series of electrical shocks playing upon its attached nerve, the record then furnished shows a similar trembling going on in the muscle as in the previous case; but the tremors of contraction are now no longer at the rate of nine per second: they correspond beat for beat with the interruptions of the electrical current. That is to say, the muscle is responding separately to every separate stimulus which it receives through the nerve; and further experiment shows that it is able thus to keep time with the separate shocks, even though these be made to follow one another so rapidly as 1,000 per second. Therefore we can have no doubt that the slow rhythm of nine per second under the influence of volitional stimulation, represents the rate at which the muscle is receiving so many separate impulses from the brain: the muscle is keeping time with the molecular vibrations going on in the cerebral hemispheres at the rate of nine beats per second. Careful tracings show that this rate cannot be increased by increasing the strength of the volitional stimulus; but some individuals—and those usually who are of quickest intelligence—display a somewhat quicker rate of rhythm, which may be as high as eleven per second. Moreover, it is found that by stimulating with strychnine any of the centres of reflex action, pretty nearly the same rate of rhythm is exhibited by the muscles thus thrown into contraction; so that all the nerve-cells in the body are thus shown to have in their vibrations pretty nearly the same period, and not to be able to vibrate with any other. For no matter how rapidly the electrical shocks are allowed to play upon the grey matter of the cerebral hemispheres, as distinguished from the nerve-trunks proceeding from them to the muscles, the muscles always show the same rhythm of about nine beats per second: the nerve-cells, unlike the nerve-fibres, refuse to keep time with the electric shocks, and will only respond to them by vibrating at their own intrinsic rate of nine beats per second.

Thus much, then, for the rate of molecular vibration which goes on in nerve-centres. But the rate of such vibration which goes on in sensory and motor nerves may be very much more rapid. For while a nerve-centre is only able to originate a vibration at the rate of about nine beats per second, a motor-nerve, as we have already seen, is able to transmit a vibration of at least 1,000 beats per second; and a sensory nerve which at the surface of its expansion is able to respond differently to differences of musical pitch, of temperature, and even of colour, is probably able to vibrate very much more rapidly even than this. We are not, indeed, entitled to conclude that the nerves of special sense vibrate in actual unison, or synchronize, with these external sources of stimulation; but we are, I think, bound to conclude that they must vibrate in some numerical proportion to them (else we should not perceive objective differences in sound, temperature, or colour); and even this implies that they are probably able to vibrate at some enormous rate.

With further reference to these molecular movements in sensory nerves, the following important observation has been made—viz. that there is a constant ratio between the amount of agitation produced in a sensory nerve, and the intensity of the corresponding sensation. This ratio is not a direct one. As Fechner states it, 'Sensation varies, not as the stimulus, but as the logarithm of the stimulus.' Thus, for instance, if 1,000 candles are all throwing their light upon the same screen, we should require ten more candles to be added before our eyes could perceive any difference in the amount of illumination. But if we begin with only 100 candles shining upon the screen, we should perceive an increase in the illumination by adding a single candle. And what is true of sight is equally true of all the other senses: if any stimulus is increased, the smallest increase of sensation first occurs when the stimulus rises one per cent, above its original intensity. Such being the law on the side of sensation, suppose that we place upon the optic nerve of an animal the wires proceeding from a delicate galvanometer, we find that every time we stimulate the eye with light, the needle of the galvanometer moves, showing electrical changes going on in the nerve, caused by the molecular agitations. Now these electrical changes are found to vary in intensity with the intensity of the light used as a stimulus, and they do so very nearly in accordance with the law of sensation just mentioned. So we say that in sensation the cerebral hemispheres are, as it were, acting the part of galvanometers in appreciating the amount of molecular change which is going on in sensory nerves; and that they record their readings in the mind as faithfully as a galvanometer records its readings on the dial.


Hitherto we have been considering certain features in the physiology of nervous action, so far as this can be appreciated by means of physiological instruments. But we have just seen that the cerebral hemispheres may themselves be regarded as such instruments, which record in our minds their readings of changes going on in our nerves. Hence, when other physiological instruments fail us, we may gain much additional insight touching the movements of nervous matter by attending to the thoughts and feelings of our own minds; for these are so many indices of what is going on in the cerebral hemispheres. I therefore propose next to contemplate the mind, considered thus as a physiological instrument.

The same scientific instinct which led Hobbes so truly to anticipate the progress of physiology, led him not less truly to anticipate the progress of psychology. For just as he was the first to enunciate the fundamental principle of nerve-action in the vibration of molecules, so was he likewise the first to enunciate the fundamental principle of psychology in the association of ideas. And the great advance of knowledge which has been made since his day with respect to both these principles, entitles us to be much more confident than even he was that they are in some way intimately united. Moreover, the manner in which they are so united we have begun clearly to understand. For we know from our study of nerve-action in general, that when once a wave of invisible or molecular movement passes through any line of nerve-structure, it leaves behind it a change in the structure such that it is afterwards more easy for a similar wave, when started from the same point, to pursue the same course. Or, to adopt a simile from Hobbes, just as water upon a table flows most readily in the lines which have been wetted by a previous flow, so the invisible waves of nerve-action pass most readily in the lines of a previous passage. This is the reason why in any exercise requiring muscular co-ordination, or dexterity, 'practice makes perfect:' the nerve-centres concerned learn to perform their work by frequently repeating it, because in this way the needful lines of wave-movement in the structure of the nerve-centre are rendered more and more permeable by use. Now we have seen that in the nerve-centres called the cerebral hemispheres, wave-movement of this kind is accompanied with feeling. Changes

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