98 INTRODUCTION TO NEUROLOGY 



nerves it may be as rapid as 125 meters per second. This rate 

 of conduction of the nervous impulse in peripheral nerves varies 

 greatly with different animals, with different nerves in the same 

 animal, and in the same nerve under different physiological con- 

 ditions. 



The reaction time required for the performance of various reflex acts 

 can be very accurately measured, and it is found that the time of even the 

 simplest reflex is considerably greater than is required for the transmission 

 of the nervous impulse through the conductors involved. The average rate 

 of conduction in human nerves is probably about 120 meters per second, and 

 the simplest reaction times which have been measured in psychological labor- 

 atories vary between 0.1 and 0.2 second (from 0.117 to 0.188 for reactions 

 to touch, and from 0.120 to 0.182 for reactions to sound). The total time 

 required for transmission of the nervous impulse through the nerve-fibers 

 involved in these reactions need not exceed 0.02 second, whence it appears 

 that the greater part of the reaction time is otherwise consumed. A part of 

 this excess time is required to overcome the inertia of the end-organs 

 (receptor and effector), and the remainder is used in the central nervous 

 system. This "central pause" is characteristic of all reflexes and, in fact, 

 has a profound significance in connection with the evolution of the higher 

 associational functions of the brain. The introduction of further complexity 

 in the reaction, of whatever sort, usually lengthens the time of the central 

 pause, though long training in making a discriminative reaction may reduce 

 this pause almost to the time of a simple reaction. 



Many attempts have been made to determine the central time of reac- 

 tions of different degrees of complexity by substracting from the total time 

 in each case the probable time required for the peripheral processes and by 

 subtracting the total time required for the simpler reactions from the total 

 time taken in more complex discriminative reactions. But further analysis 

 (particularly more critical introspection) has shown that in these human 

 reactions the problem is too complex to be resolved by this method (see 

 Ladd and Woodworth, 1911, p. 497). 



The simpler reflexes of lower vertebrates can be studied physiologically, 

 and these give data which are much more readily analyzed than the more 

 complex human reactions. In the case of the simplest reflex obtainable in 

 the spinal cord of the frog, the central pause was estimated by Wundt to be 

 only 0.008 second, i. c., all of the time required for the reaction except this 

 interval was used in the peripheral apparatus. But in a crossed reflex, 

 where the reaction occurs on the opposite side of the body from the stimu- 

 lus, the increased complexity of the central process consumed 0.004 second 

 additional. 



Miss Buchanan (1908), with more accurate methods of study, finds in the 

 frog that the central time varies between .014 and .021 second. She also 

 measured the additional latent time required for a crossed reflex, and found 

 it to be of the same order of magnitude MS the latent time of the simple 

 reflex (instead of half as much as in Wundt's experiments), that is, the 

 crossed reflex required about twice the l.-itent time in the spinal cord as the 

 uncrossed reflex. It is assumed that this central pause in the uncrossed 

 reflex is consumed chiefly in the synapses between the peripheral sensory 

 and the peripheral motor neurons, and that only one such synapse is in- 

 volved in each simple reflex connection (a two-neuron circuit, see Fig. 1, 



