Page:Observations on Man 1834.djvu/78

 organ. And of miniature vibrations belonging to this region, such as b, c, d, &c. it is evident, that b has the preference, since A leans to it a little, even in its own primary seat, more and more, in receding from this, and almost entirely, when it comes to the primary seat of B. For the same reasons B impressed alone will excite a; and, in general, if A, B, C, &c. be vibrations synchronically impressed on different regions of the medullary substance, A impressed alone will at last excite b, c, &c. according to the proposition.

If A and B be vibrations impressed successively, then will the latter part of A, viz. that part which, according to the third and fourth propositions, remains, after the impression of the object ceases, be modified and altered by B, at the same time that it will a little modify and alter it, till at last it be quite overpowered by it, and end in it. It follows therefore, by a like method of reasoning, that the successive impression of A and B, sufficiently repeated, will so alter the medullary substance, as that when A is impressed alone, its latter part shall not be such as the sole impression of A requires, but lean towards B, and end in b at last. But B will not excite a in a retrograde order; since, by supposition, the latter part of B was not modified and altered by A, but by some other vibration, such as C or D. And as B, by being followed by C, may at last raise c; so b, when raised by A, in the method here proposed, may be also sufficient to raise c; inasmuch as the miniature c being a feeble motion, not stronger, perhaps, than the natural vibrations N, requires only to have its kind, place, and line of direction, determined by association, the heat and arterial pulsation conveying to it the requisite degree of strength. And thus A impressed alone will raise b, c, &c. in successive associations, as well as in synchronous ones, according to the proposition.

It seems also, that the influence of A may, in some degree, reach through B to C; so that A of itself may have some effect to raise c, as well as by means of b. However, it is evident, that this chain must break off, at last, in long successions; and that sooner or later, according to the number and vigour of the repeated impressions. The power of miniature vibrations to raise other miniatures may, perhaps, be made clearer to mathematicians, by hinting, that the efficacy of any vibration to raise any other, is not in the simple ratio of its vividness, but as some power thereof less than unity; for thus b may raise c, a weaker vibration than b, c may raise d, &c. with more facility than if the efficacy was in the simple ratio of the vividness, and yet so that the series shall break off at last.

If the ninth proposition be allowed, we may prove this in somewhat a shorter and easier manner, as follows. Since the vibrations A and B are impressed together, they must, from the diffusion necessary to vibratory motions, run into one vibration; and consequently, after a number of impressions sufficiently