AND ITS MODE OF ACTION. 431 



first, the time requisite for the mechanism of muscular contraction, 

 namely, 0.0 1", and, secondly, that occupied in the transmission of 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 Ilelmholtz, 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 in the 

 external, middle, and internal parts of the ear. The whole process, 

 therefore, of executing a voluntary movement in the foot, at the signal 

 given by a bell, would be divided in time as follows : 



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



Mechanism of hearing 0.010" 



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

 Transmission through the spinal cord . . . . 0.044" 



Transmission through the sciatic nerve .... 0.044" 

 Mechanism of muscular contraction ..... 0.010" 



0.220" 



It appears that the nervous action in the brain, which represents the 

 operation of the gray substance of the nervous centres, requires a con- 

 siderably longer time than the transmission of a nervous impulse through 

 the nerve fibres. 



The physiological variation in rapidity of any or all the nervous 

 actions above enumerated, in different individuals, causes a difference 

 in the promptitude with which sensible phenomena are perceived and 

 recorded by different observers. This fact was first distinctly noticed 

 in astronomical observations, where it was found that the exact time 

 of the passage of a star across the thread of a transit instrument was 

 differently recorded by different observers ; this variation in some cases 

 amounting to as much as one second. Subsequent observations showed 

 that in no case was the time of the transit recorded with absolute 

 accuracy ; but that a certain delay always intervened, due to the time 

 necessarily occupied by the nervous mechanism of the observer. This 

 fact was established by imitating the transit of a star by means of a 

 single luminous point moving in a circle with uniform velocity before 

 the field of a telescope. By contrivances similar to those described 

 above, the real instant of the passage of this luminous point across the 

 thread of the telescope field was recorded upon a revolving cylinder, 

 and the observer also marked its passage by similar means. The 

 difference between the real and the observed time represented the 

 " personal error" of the observer. The amount of this error, however, 

 although it varies for different persons, is constant, or nearly so, for the 

 same individual ; and, when it has been once ascertained, the results of 

 observation may be so corrected as to r.pproach nearly to absolute 

 precision. 



