NERVOUS IRRITABILITY. 369 



Finally, the rate of transmission of the nerve-force, in man, for both 

 voluntary motion and conscious sensation, has been investigated by 

 Burckhardt,* with a registering apparatus in which the beginning and 

 end of the nervous transmission were marked, as above, by the devia- 

 tions of a traced line. 



Rate of Transmission in the Motor Nerves, The transmission of 

 the voluntary impulse was measured by Burckhardt as follows : The 

 apparatus being attached to the person serving for experiment, the 

 signal for voluntary motion was given by a bell connected with the 

 battery. Thus the entire interval registered was that between the 

 sound of the bell and the muscular contraction. A part of this time 

 was consumed in hearing the sound and producing the volitional 

 impulse. Another part was taken up by the process of muscular con- 

 traction ; and only the remainder was occupied by nervous transmission. 

 But when, in different observations, the same signal was used for 

 Ihe contraction of muscles supplied by different lengths of nerve, the 

 processes taking place in the brain and in the muscle would be alike 

 in all ; and any difference in the time observed must be due to the 

 different lengths of nerve traversed by the motor impulse. The 

 muscles employed for this purpose were, in the lower limb, the 

 extensor digitorum communis brevis, tibialis anticus, and semimem- 

 branosus, supplied by branches of the sciatic nerve, and the quadriceps 

 extensor cruris, supplied by the anterior crural nerve; in the upper 

 limb, the interosseus externus primus, extensor digitorum communis, 

 flexor digitorum and deltoid, all supplied by branches of the brachial 

 plexus. The mean result of these observations, on eight healthy 

 persons, gave a velocity of transmission, in the nerves of the upper 

 and lower limbs, of a little over 27 metres per second. The minimum 

 velocity was 20 metres, and the maximum 36 metres ; but of all the 

 observations, thirty in number, twenty-three, or nearly four-fifths, gave 

 results between 26 and 28 metres. 



In one instance the rate of movement for the voluntary impulse and 

 for that excited by galvanism was tested in fne same nerve, with but 

 little difference in the results. 



According to Burckhardt, furthermore, the rate of transmission does 

 not vary essentially for weak or strong motor impulses ; that for a mus- 

 cular contraction of moderate force passing as rapidly through the 

 nerve as that for contractions of greater power. 



Rate of Transmission in the Sensitive Nerves. The rate of trans- 

 mission for impressions of conscious sensibility is determined by a simi- 

 lar method. A tactile impression is made upon the skin at varying 

 distances from the nervous centre as, for instance, upon the foot, the 

 thigh, and the loins ; the instant at which the sensation is perceived 

 being indicated by a movement of the finger. As the time required 

 for conscious perception in the brain and for voluntary movement of 



* Die Physiologische Diagnostik der Nervenkranheiten. Leipzig, 1875, p. 32. 



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