6 1 4 THE NERVE CELL. 



column of the cord, no block is met with, and the nerve impulses can 

 pass freely in either direction. 



Refractory period of nerve cells. — When two successive stimula- 

 tions are applied to the same sensory nerve cell, nerve fibre, or nerve 

 termination, they are only appreciated as two, if a certain small 

 interval of time has elapsed between their application ; in other 

 words, the nerve substance only responds to a second stimulus after a 

 certain interval has elapsed since the removal of the first. This is 

 a general phenomenon of living substance, and is quite similar 

 in character to the refractory period of cardiac and of skeletal 

 muscle. The difference is found to be by no means the same for 

 different terminal organs, being very small (0*002 second) in the case of 

 the auditory organ (noise of two successive sparks), much longer 

 (0*017 second) in the case of the eye (image of two successive sparks 

 falling on the same spot of the retina), and still longer (0*0277) in the 

 case of the sense of touch (two successive touches of the skin of the 

 finger). The relative amount of this so-called " minimal difference " in 

 these cases is in all probability merely a measure of the relative inertia 

 of the different kinds of terminal apparatus. This difference of inertia 

 is probably also in a measure responsible for the fact that if stimuli be 

 applied simultaneously to two different sense organs, as when a sound 

 is emitted at the same instant that a spark is rendered visible, or at 

 the same moment that the skin is excited, the two sensations are not 

 appreciated simultaneously ; in order that they should be so, the one 

 must operate a certain time sooner than the other. The difference is 

 known as the " differential time " between two senses. In this case, the 

 difference may be partly due to a difference in the time taken by the 

 nerve impulses in traversing the nerve paths, and this, again, is 

 probably mainly due to a difference in the number of synapses which 

 have to be passed in the two cases. 



The following measurements are taken from Exner : x — 



Smallest possible interval between two stimuli to produce two distinct 

 sensations. 



Hearing (two electrical sparks) . . . 0*002 second. 

 Retinal excitation (direct application of elec- 

 trodes to eye) . . . . . . 0*017 ,, 



Retinal excitation (by optical images on 



fovea) 0044 „ 



Tactile (two successive touches on finger) . 0*028 ,, 



Sight and touch ...... 0*050 ,, 



Sight and hearing ..... 0*060 ,, 



Richet 2 has attempted directly to determine the refractory period 

 of the nerve cell of the motor cerebral cortex in the anaesthetised dog. 

 He found that, if stimuli succeed one another at a rate of 10 per second, 



1 Hermann's "Handbuch," Bd. ii. S. 262, where further details regarding the differential 

 time, and also the reaction time, will be found. See also Exner, "Exper. Untersnch. d. 

 einfachsten psychischen Processe," Arch. f. d. ges. Physiol., Bonn, Bd. vii. and xi. ; 

 Wundt, " Neuere Leistungen auf d. Gebiete der physiol. Psychologie," Vrtljschr. f. 

 Psychiat., Neuwied, 1867, Bd. i. ; Donders, " Schnelligkeit psychischer Processe," Arch, 

 f. AnaL, Physiol., u. loissensch. Med., 1868 ; Kries and Anerbach, Arch. f. Physiol., 

 Leipzig, 1877 ; Richet, " Dictionnaire de physiologie," loc. cit., 1899. 



-Rep. Brit. Ass. Adv. Sc, London, 1897. Lecture to British Association in Nature, 

 Oct. 26, 1899. 



