ON reaction-times and the velocity of the nervous IMPULSE.1 
By J. McKehn Cattkll, Ph. D., and Charles S. Dolley, M. D. 
The object of this research is to determine the conditions which affect the length of the 
reaction-time on dermal stimuli, and to study the application of the reaction-time to the measure¬ 
ment of the velocity of the nervous impulse in motor and sensory nerves and in motor and sensory 
tracts of the spinal cord. 
Since von Helmholtz first measured the velocity of the nervous impulse in 1850 much work 
has been directed to the subject, but the results are not accordant. The experiments on the 
nerve-muscle preparation of the frog are the most easily carried out, and these are usually 
regarded as valid for the motor and sensory nerves of man. It does not, however, follow that 
the effects of electrical stimulation on the excised and dying nerve of the frog are the same as the 
effects of cerebral discharge in the living animal, nor that these effects (could they be determined) 
would hold for man. 
Determinations made by electrically stimulating the living nerve of the lower mammals and 
of man are of more value for human physiology than those on the excised nerve of a frog. They 
are, however, less accordant. We are ignorant of the relations between electrical stimulation 
and nervous discharge, and do not know what happens in the motor nerve and muscle when the 
skin is stimulated by electricity. It seems evident that the velocity of the normal nervous impulse 
can not be determined iu this way, owing to the great variation in results, which must be due to 
the method of stimulation and not to the A-elocity of the normal impulse. Thus von Helmholtz 
obtained times twice as long in winter as in summer, and supposes this to be due to differences 
in the conductivity of the nerve. This is not, however, the case, as we find that the reaction¬ 
time, in which the time of transition along the nerves is a large factor, is the same in winter as 
in summer. It is further evident that such experiments apply only to the motor nerve. The time 
of transmission may be the same as in the sensory nerve, but to assume this would be arbitrary. 
So long as we can not record the progress of the nervous impulse along the nerve nor the 
instant at which it reaches or leaves the brain, the rate of transmission of the normal sensory or 
motor impulse can only be determined indirectly. In the case of motor nerves it is necessary to 
make movements with muscles at varying distances from the brain following as quickly as possible 
on the same stimulus. In the case of sensory nerves the stimulation must be given at varying 
distances from the brain, and the arrival must be followed by a movement or directly judged by 
consciousness. 
In these experiments the results are obtained by measuring the time of a complex process— 
the reaction. The reaction-time is the interval elapsing before a predetermined movement follows 
on a predetermined stimulus. During this interval a series of physiological processes takes 
place. (1) The stimulus is converted into a nervous impulse; (2) the nervous impulsé travels 
along the sensory nerve and, it may be, the spinal cord to the brain; (3) through sensory tracts of 
the brain to a sensory center; (4) changes occur in this center; (5) these changes are followed by 
Presented by Prof. Ci. F. Barker before tlie meeting of the National Academy of Sciences, Albany, 1893. 
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