Page:Mind (Old Series) Volume 11.djvu/234

THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 233 impulse follows a line of small resistance to the centre for the coordination of motions, and the proper nervous impulse, already prepared and waiting for the signal, is sent from the centre to the muscle of the hand. When the reaction has often been made the entire cerebral process becomes automatic, the impulse of itself takes the well-travelled way to the motor centre, and re- leases the motor impulse. 1 I now go on to give the results of my experiments. I only give the determinations made on B (Dr. G. O. Berger) and C (the writer) ; I have made similar determinations on other sub- jects of different age, sex, occupation, etc., but these can be better considered after we know the results of careful and thorough experiments on practised observers. We have first to consider the simple reaction-time for light. When this was to be measured, all being in readiness, as described in the foregoing section, the experimenter said ' Jetzt,' and the observer fixated the point afc which the light was to appear, and put himself in readiness to make the reaction. The experimenter then set the clock-work of the chronoscope in motion, and about one second afterwards caused the light to appear by means of the apparatus described. The observer lifted his hand as soon as possible after the appear- ance of the light, and the interval that had elapsed between the occurrence of the light and the commencement of the muscular contraction was read by the experimenter directly from the chronoscope. In no single case, as far as I can remember, did the observer make a premature reaction, that is, lift his hand be- fore the necessary physiological operations had had time to occur. The only disturbance was caused by the clock-work of the chronoscope sometimes not being properly controlled by the vibrating spring. If the experimenter noticed this in time he did not produce the light. This occasional failure of the chronoscope was always noticed, so does not interefere with the accuracy of the times here given, but the observer was sometimes disturbed so that his reactions may have been made less regular. Through- out this paper I give every series and every reaction made ; I give, however, in addition to each series, a corrected value reached by the method above described. This correction simply excludes all abnormal values. In the Tables I give the average of the variation of each reaction from the average of the series to which it belongs (V) ; that is, if A is the average of the n reactions 1 This theory concerning the nature of the reaction would be none the less probable, though we suppose the centres for sensation and perception not to be distinct, or indeed that in the reaction the brain, in some mysterious way, ' acts as a whole '. In this paper I take it for granted throughout that mental states are due to changes in the brain. We know, however, but little as to the functions of the brain. I therefore make as few assumptions as possible, and these must be kept apart from the positive results, which it is the object of this paper to make known. 16