NERVOUS IRRITABILITY. 371 



on the other hand, was found to be nearly as rapid as through the 

 nerves, the average rate being a little over 42 metres per second. 

 There was a remarkable difference, however, in this respect, between 

 tactile impressions and those of a painful character. The latter are 

 transmitted at a much slower rate, amounting on the average to not 

 more than 13 metres per second. Thus the transmission of motor 

 impulses and of tactile and painful impressions respectively, through 

 the spinal cord, is as follows : 



KATE OF TRANSMISSION THROUGH THE SPINAL CORD. 

 For tactile impressions . . . .42 metres per second. 



'' painful 13 



motor impulses 10 " 



According to these results the passage of a motor impulse, from the 

 brain to the muscles of the foot, would occupy 0.088 of a second ; of 

 which about one-half would be required for transmission through the 

 spinal cord, and one-half for transmission through the nerves. 



Rapidity of Xervous Action in the Brain. In the above experi- 

 ments, an essential part of the nervous operation consists in hearing 

 the signal for a voluntary movement and in the volition which gen- 

 erates the motor impulse. The time thus consumed is ascertained by 

 deducting, from the whole period between a given signal and a vol- 

 untary movement, first, the time requisite for muscular contraction, 

 namejy, 0.01"; and, secondly, that occupied in transmitting the impulse 

 through the spinal cord and nerves. Thus if the entire period be 

 0.220", and the time required for transmission through the spinal 

 cord and nerves be 0.088", there remains 0.132", which is occupied in 

 muscular contraction and in the acts of sensation and volition. Burck- 

 hardt's experiments, like those of Helmholtz,' fix the time required 

 for local stimulation of the muscle at 0.01"; and he estimates that 

 about an equal interval is necessary for the mechanism of hearing. 

 The whole process, therefore, of executing a voluntary movement in 

 the foot, at the signal of a bell, would be divided as follow.- : 



TIME OCCUPIED IN EXECUTING A VOLUNTARY MOVEMENT AT A GIVEN SIGNAL. 

 Mechanism of hearinir ....... 0.010" 



Acts of perception and volition in the brain . . . 0.112" 

 Transmission throuirh the spinal cord .... 0.044" 



Transmission through the sciatic nerve .... 0.044" 



Mechanism of muscular contraction 0.010" 



0.220" 



It appears that the action in the brain, representing the operation 

 of the gray substance of the nervous centres, requires a consider- 

 ably longer time than the transmission of impulses through the nerve 

 fibres. 



Personal Error and Personal Equation. The different rapidity of 

 nervous action, in different individuals, causes a variation in the prompti- 



