516 PHYSIOLOGY 



When the leg is drawn up in response to a painful or nocuous 

 stimulus applied to the foot, a certain amount of time is involved 

 in each of the following links in the chain of processes which determine 

 the reaction : 



(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 

 grey matter of the cord. 



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

 the spinal cord to the muscles. 



(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. 

 (2), (4). (5), and (6) represent quantities for the measurement of which 

 we have all the necessary data. 



In any given reflex therefore we may add these periods together 

 and subtract them from the total reaction time ; we thus get a 

 ' reduced reaction time,' which represents the time involved in the 

 passage of the impulse through the central nervous system, and 

 in the conversion of an afferent impulse into an aggregate of co- 

 ordinated motor impulses. If 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 which it is conducted by the same nerve 

 fibre outside the cord, the extra delay which occurs in the passage 

 of the impulse through the cord must take place either in the nerve- 

 cells themselves, or in the synapses, through which the impulse 

 passes from one neuron to the next in the chain of reflex elements. 



The rate of passage of an impulse through the nerve-cell can 

 only be determined in one part of the body, viz. in the posterior 

 spinal root ganglia, since only in these is it possible to detect the 

 moment of passage of an impulse across a given section of a nerve 

 fibre on both sides of the ganglion-cell in which the nerve fibres arise. 

 Experiments on this point have been made by Steinach and by 

 Moore. In each case the time occupied in the passage of the impulse 

 through the ganglion was not appreciably longer than if the impulse 

 had passed through a corresponding stretch of uninterrupted nerve 

 fibre. We are therefore justified in concluding that the relatively great 

 delay in the passage of an impulse through the central nervous system 

 has its seat in the synapses across which the impulse has to pass. 



