132 MENTAL EVOLUTION IN ANIMAL& 



tion — or, more correctly, on what is known touching the 

 physiological processes which accompany Perception. 



In earlier chapters I have already stated that the only 

 distinction which is known on the physiological side between 

 a nervous activity which is accompanied by consciousness, 

 and a nervous activity which is not so accompanied, consists 

 in a difference of time. I shall now give the experimental 

 data on which the statement rests. 



Professor Exner has determined the time which is occu- 

 pied by a nerve-centre of man in executing its part in the 

 performance of a reflex action. That is to say, the rate of 

 transmission of a stimulus along a nerve being known, and 

 the length both of the afferent and efferent nerves concerned 

 in a particular reflex act being known, as also is the 

 " period of latency " of a muscle ; the time occupied by the 

 nerve-centre in conducting its operations was determined by 

 subtracting the time occupied by the passage of the stimulus 

 along the afferent and efferent nerves, plus the period of 

 latency of a muscle, from the total time between the fall of 

 the stimulus and the occurrence of the muscular contraction. 

 This time was found in the case of the reflex closure of the 

 eye-lid to vary between O0471 and - 0555 of a second 

 according to the strength of the stimulus.* By a similar 

 process Exner has estimated the time required for the central 

 nervous operations which are together comprised in having a 

 simple sensation, perceiving the sensation, and the volitional 

 act of signalling the perception. That is to say, an electrical 

 shock being administered to one hand, and as quickly as 

 possible signalled by the other, the time occupied by the 

 nerve-centre in performing its part of the process was esti- 

 mated as in the previous case. This time in the case of this 

 experiment was found to be O0828", which is nearly twice as 

 long as that which, as we have just seen, is required for a 

 nerve-centre to perform its part in a reflex aetion.-f* 



Acts of perception in which different senses are concerned 

 occupy different times. This interesting topic has been 

 investigated by a number of physiologists.^ According to 

 Donders the total " reaction-time " (i.e., between stimulus 

 and response) is, roughly speaking, for touch \, for hearing £, 



* Arch./, d. ges. Physiol., xliii, 526 (1874). 



f Ibid., vii, p. 610. ' 



X See Herman, Eandb. d. Physiol., Bd. II, Tli. 2, 8. 264. 



