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PHYSIOLOGY 



( 1 ) The conversion in the peripheral sense organ of the mechanical 

 stimulus into a nerve process. 



(2) The passage of a nerve impulse up the nerve from the end organ to the 

 spinal cord. 



(3) The passage of the impulse across two or more synapses in the urcy 

 matter of the cord. 



(4) The passage of the impulse down the motor nerve fibres from the 

 spinal cord to the muscles. 



FIG. 236. Arrangement of apparatus for determination of reaction time. 



(ALCOCK and ELLISON.) 



R, coil ; E, exciting electrodes ; F, tuning-fork ; A, B, keys ; s, T, electro- 

 magnetic signals ; D, drum. 



(5) The processes occurring in the end 'organs of the muscle. 



(6) The latent period in the muscle fibre itself. 



With a weak stimulus No. 1 is impossible to measure. With a strong 

 stimulus it may be so short as to be practically negligible. |'J), (-1), (5) 

 and (6) represent quantities for the measurement of whirli we have all the 

 necessary data. 



In any given reflex therefore we may add these periods together and 

 subtract them fnm the total reaction time; \ve thus get a 'reduced re- 

 act inn time,' which represents the time involved in the passage of the impulse 

 throiiL'h the central nervous system, and in the conversion of an alTerent 

 impulse into an aggregate <! co-ordinated motor impulses. It is found 

 that the reduced reaction time accounts for the greater part of the total 

 reaction time. Since we have no reason to assume that the rate of passage of 

 an impulse through the intra-spinal course of a nerve fibre differs appreciably 

 from the rate at \\hidi it ix conducted by the same nerve fibre outside the 



