in the central or in the peripheral organ. Let A, fig. 3, represent a peripheral organ, B a central organ, and C the nerve connecting them. Let a stimulus, E, applied at A evoke a sensation, S, at B. Similarly, let another stimulus, E-_>, applied at A evoke a corresponding sensation, 82, at B. Then the experimental facts may be stated thus : if there is a certain time-interval of ^ of a second between EI and E 2 , and a certain relation between them of intensity and duration, the sum of the resulting sensations Si and So may be practically zero, or, at any rate, very much smaller than either sensation by itself.
FIG. 3.
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(1.) The first obvious deduction is that made by Exner, viz., that a period of at least .y must elapse between the application of the stimulus at A and the arousing of the sensation at B.
(2.) It is also evident that this delay of T V is not simply the transmission time of the nerve-response along C from A to B, for, if it were, the second impulse would reach B too late to affect the first, unless we suppose either a double track along C or the propagation along a single track of a double response, with components travelling at different rates a hypothesis not supported by any facts within my knowledge.
(3.) If the delay of T \/' occcurs at B we must also suppose either it double track along C, or else that each nerve-fibre is capable of transmitting two opposite kinds of response, namely, one with a latency of 7 y arousing sensation, and the other deadening it, and acting without delay.
(4.) It seems simpler to suppose that this delay of r y occurs some- where on the track between A and B, and probably close to A, e.g., at D, and that it is quite distinct from any delay due to transmission.
(5.) Not only do these experiments prove that there is a delay, but that, under certain circumstances, a second stimulus may modify to the point of annulling it the response to a previous stimulus. It has been shown* that two stimuli in rapid succession may give rise in nerve to what appears to be a single response, but in that case the response is at least as strong as either stimulus would produce alone, whereas here the response is less than it would have been without the second stimulus.
These facts point to the existence, probably in the retina, of some mechanism sensitive to light, the function of which is to regulate for
- G otcti and Burch, ' Journal of Physiology,' vol. 24, p. 410.
