458 



PHYSIOLOGY 



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, 



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. 



since only in these is it possible to detect the moment of passage of an im- 

 pulse 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. This con- 

 clusion is in accordance with our experience on the latent period of muscle, 

 the greater part of which is due to changes occurring in the nerve-endings, 

 i.e. in the synapses between motor nerve and muscle. The greater the 

 number of synapses involved in any given reaction, i.e. the greater the com- 

 plexity of the reaction, the longer will be the period which elapses between 

 the moment of application of the stimulus and the moment at which the 



