668 RAPIDITY OF CONDUCTION OF THE STIMULUS IN NERVES. 



of the muscle, the lever H falls so that the point e rests upon the underlying fixed 

 plate y, the contact at f nevertheless remains open, and, therefore, also the galva- 

 nometer-circuit. Another method is described on p. 654. 



In man v. Helmholtz and Baxt determined the propagation- velocity of the 

 impulse in the median nerve by having the musculature of the thenar eminence 

 record its contraction by means of a lever upon a rapidly rotating cylinder. The 

 irritation of the nerve was practised on one occasion in the axillary cavity and 

 on the second at the wrist-joint. The contraction-curves, naturally, exhibited 

 differences as to the moment of beginning. The difference in the time-value for 

 these two gives the time for the conduction in the intervening nerve-segment. In 

 the experiment the entire arm is enclosed in a plaster bandage in order to secure 

 rest of the muscles of the arm. 



According to Bernstein, the time necessary for the stimulus that passes 



from the motor nerve to the 

 muscle to excite the motor 

 nerve-endings is on the 

 average 0.0032 second in 

 the frog and 0.0015 second 

 in warm-blooded animals. 



In the sensory nerves 

 of man the impulse is 

 probably propagated 

 with the same rapidity 

 as in the motor nerves. 

 The figures obtained 

 vary, it is true, between 

 the wide limits of 94 and 

 30 meters in a second. 



Method of Examination. 



In the person under ob- 

 servation two points at as 

 widely unequal distances 

 from the brain as possible 

 are irritated for a moment 

 in succession, for example 

 the lobule of the ear and the 

 great toe, as by an open- 

 ing induced current. The 

 moment of irritation is 



noted, for example by beginning the vibrations of a tuning-fork plate, the removal 

 of the clamp from the tuning-fork at the same time opening the primary current. 

 The person examined should in each instance indicate by an appropriate sign upon 

 the registering surface the time when irritation is perceived. The reaction-time to 

 be taken into consideration in this connection is discussed on p. 777. 



A needle-prick of the skin causes at first the sensation of pricking. There 

 then follows an interval free from sensation and finally again a pricking sensation, 

 apparently originating from within. 



Pathological. In the presence of disease of the spinal cord the remarkable 

 observation has occasionally been made of a striking retardation of conduction 

 in the sensory nerves of the skin. The sensation itself may, under such circum- 

 stances, be unaltered. Occasionally only the conduction of painful sensations 

 was found to be retarded, so that a painful impression upon the skin was first 

 perceived only as a tactile sensation and then as pain, or conversely. If the 

 interval of time between these two impressions is considerable, there may be 

 well-defined double sensation. 



In the sphere of the motor nerves retarded conduction for voluntary movement 

 is observed, for example in cases of senile paralysis agitans. The observation has 

 been made, further, in rare cases that with otherwise well-developed musculature, 

 voluntary movements were executed much more slowly, as the interval between 

 the impulse of the will and the contraction was prolonged, and, besides, the muscles 

 occupied a longer time in contracting, a sort of tonic contraction thus resulting. 

 In nerves exhibiting degenerative reactions the retardation of conduction in 



W 



FIG. 235. v. Helmholtz's Method for Determining the Propagation- 

 velocity of the Nerve-stimulus. 



