Part 9 (2/2)

[Ill.u.s.tration: FIG. 65.--RESPONSES OF A WIRE TO AMPLITUDES OF VIBRATION 45 AND 90 (_a_) Responses when the wire was in a sluggish condition at temperature of 5 C.

(_b_) Enhanced response at 30 C.

(_c_) Diminution of response at 90 C.]

#Diphasic variation.#--It has already been said that if two points A and B are in the same physico-chemical condition, then a given stimulus will give rise to similar excitatory electric effects at the two points. If the galvanometer deflection is 'up' when A alone is excited, the excitation of B will give rise to a downward deflection. When the two points are simultaneously excited the electric variation at the two points will _continuously_ balance each other. Under such conditions there will be no resultant deflection. But if the intensity of stimulation of one point is relatively stronger, then the balance will be disturbed, and a resultant deflection produced whose sign and magnitude can be found independently by the algebraical summation of the individual effects of A and B.

It has also been shown that a balancing point for the block, which is approximately near the middle of the wire, may be found so that the vibrations of A and B through the same amplitude produce equal and opposite deflection. Simultaneous vibration of both will give no resultant current; when the block is abolished and the wire is vibrated as a whole, there will still be no resultant, inasmuch as similar excitations are produced at A and B.

After obtaining the balance, if we apply an exciting reagent like Na_2CO_3 at one point, and a depressing reagent like KBr at the other, the responses will now become unequal, the more excitable point giving a stronger deflection. We can, however, make the two deflections equal by increasing the amplitude of vibration of the less sensitive point. The two deflections may thus be rendered equal and opposite, but the time relations--the latent period, the time rate for attaining the maximum excitation and recovery from that effect--will no longer be the same in the two cases. There would therefore be no continuous balance, and we obtain instead a very interesting diphasic record. I give below an exact reproduction of the response-curves of A and B recorded on a fast-moving drum. It will be remembered that one point was touched with Na_2CO_3 and the other with KBr. By suitably increasing the amplitude of vibration of the less sensitive, the two deflections were rendered approximately equal. The records of A and B were at first taken separately (fig. 66, _a_). It will be noticed that the maximum deflection of A was attained relatively much earlier than that of B.

The resultant curve R' was obtained by summation.

[Ill.u.s.tration: FIG. 66.--DIPHASIC VARIATION (_a_) Records of A and B obtained separately. R' is the resultant by algebraical summation. (_b_) Diphasic record obtained by simultaneous stimulation of A and B.]

After taking the records of A and B separately, a record of resultant effect R due to simultaneous vibration of A and B was next taken. It gave the curious two-phased response--positive effect followed by negative after-vibration, practically similar to the resultant curve R'

(fig. 66, _b_).

The positive portion of the curve is due to A effect and the negative to B. If by any means, say by either increasing the amplitude of vibration of A or increasing its sensitiveness, the response of A is very greatly enhanced, then the positive effect would be predominant and the negative effect would become inconspicuous. When the two const.i.tuent responses are of the same order of magnitude, we shall have a positive response followed by a negative after-vibration; the first twitch will belong to the one which responds earlier. If the response of A is very much reduced, then the positive effect will be reduced to a mere twitch and the negative effect will become predominant.

I give a series of records, fig. 67, in which these three princ.i.p.al types are well exhibited, the two contacts having been rendered unequally excitable by solutions of the two reagents KBr and Na_2CO_3. A and B were vibrated simultaneously and records taken.

(_a_) First, the relative response of B (downward) is increased by increasing its amplitude of vibration. The amplitude of vibration of A was throughout maintained constant. The negative or downward response is now very conspicuous, there being only a mere preliminary indication of the positive effect. (_b_) The amplitude of vibration of B is now slightly reduced, and we obtain the diphasic effect. (_c_) The intensity of vibration of B is diminished still further, and the negative effect is seen reduced to a slight downward after-vibration, the positive up-curve being now very prominent (fig. 67).

[Ill.u.s.tration: FIG. 67.--NEGATIVE, DIPHASIC, AND POSITIVE RESULTANT RESPONSE]

#Continuous transformation from negative to positive.#--I have shown the three phases of transformation, the intensity of one of the const.i.tuent responses being varied by altering the intensity of disturbance.

In the following record (fig. 68) I succeeded in obtaining a continuous transformation from positive to negative phase by a continuous change in the relative sensitiveness of the two contacts.

I found that traces of after-effect due to the application of Na_2CO_3 remain for a time. If the reagent is previously applied to an area and the traces of the carbonate then washed off, the increased sensitiveness conferred disappears gradually. Again, if we apply Na_2CO_3 solution to a fresh point, the sensitiveness gradually increases. There is another further interesting point to be noticed: the beginning of response is earlier when the application of Na_2CO_3 is fresh.

[Ill.u.s.tration: FIG. 68.--CONTINUOUS TRANSFORMATION FROM NEGATIVE TO POSITIVE THROUGH INTERMEDIATE DIPHASIC RESPONSE Thick dots represent the times of application of successive stimuli.]

We have thus a wire held at one end, and successive uniform vibrations at intervals of one minute imparted to the wire as a whole, by means of a vibration head on the other end.

Owing to the after-effect of previous application of Na_2CO_3 the sensitiveness of B is at the beginning great, hence the three resultant responses at the beginning are negative or downward.

Dilute solution of Na_2CO_3 is next applied to A. The response of A (up) begins earlier and continues to grow stronger and stronger. Hence, after this application, the response shows a preliminary positive twitch of A followed by negative deflection of B. The positive grows continuously. At the fifth response the two phases, positive and negative, become equal, after that the positive becomes very prominent, the negative being reduced as a feeble after-vibration.

It need only be added here that the diphasic variations as exhibited by metals are in every way counterparts of similar phenomena observed in animal tissues.

CHAPTER XIV

INORGANIC RESPONSE--FATIGUE, STAIRCASE, AND MODIFIED RESPONSE

Fatigue in metals--Fatigue under continuous stimulation--Staircase effect--Reversed responses due to molecular modification in nerve and metal, and their transformation into normal after continuous stimulation--Increased response after continuous stimulation.

[Ill.u.s.tration: FIG. 69.--FATIGUE IN MUSCLE (WALLER)]

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