AND ITS MODE OF ACTION. 429 



lower extremities, is a little over 2t metres per second. The minimum 

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

 observations, which were thirty in number, twenty-three, or nearly four- 

 fifths, gave results between 26 and 28 metres. 



In one instance the rate of movement in the same nerve, for the 

 voluntary impulse and for that excited by galvanism, was tested com- 

 paratively, and but little difference was found to exist in the two cases. 



According to Burckhardt, also, the rate of transmission does not vary 

 essentially for weak or strong motor impulses; that for a muscular con- 

 traction of moderate force passing as rapidly through the nerve as that 

 for contractions of greater power. 



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

 mission for impressions of conscious sensibility is determined by an 

 analogous method. An irritation or tactile impression is produced 

 upon the skin at varying distances from the nervous centres as, for 

 instance, upon the foot, the thigh, and the loins ; and the instant at 

 which the sensation is perceived is indicated by a movement of the 

 finger. As the time required for the act of conscious perception in 

 the brain and for the voluntary movement of the finger is the same 

 in all cases, the difference between two successive observations is owing 

 to the different lengths of the nerves transmitting the stimulus. 



In the investigations of Burckhardt, which were made upon thirteen 

 different individuals, the mean rate of transmission for sensitive impres- 

 sions through the peripheral nerves was found to be a little less than 

 47 metres per second; that is, more than one and a half times as rapid 

 as that for voluntary motion. The variations were from a minimum 

 of 20 to a maximum of 73 metres; but in nearly three-fourths of all the 

 observations, the results were confined within a variation of from 40 to 

 56 metres. The rapidity of transmission varied but little with the in- 

 creased or diminished intensity of the impression; the difference, on 

 the average, being but little over one per cent. 



Eate of Transmission in the Spinal Cord. The investigations of 

 Burckhardt first indicated a difference between the rate of transmission 

 in the spinal cord and that in the peripheral nerves. This rate was 

 determined for the spinal cord by comparing the time consumed in 

 the passage of a voluntary impulse to the extremities of two nerves, 

 like the sciatic and the ulnar, which emerge from the spinal cord at 

 different points. In this case the voluntary impulse, after leaving the 

 brain, will traverse different lengths of the spinal cord ; and as its rate 

 of movement in the peripheral nerves is known, the difference in the 

 time of its entire passage may be easily referred to its increased or 

 diminished rate of movement in the spinal cord. Thus a motor impulse, 

 which calls into action the interosseous muscles of the hand, passes 

 through the cervical portion of the spinal cord, and thence through the 

 lower cervical nerves, the brachial plexus^ and the whole length of the 

 ulnar nerve. An impulse which excites contraction in the quadriceps 

 extensor cruris passes through both the cervical and dorsal portions of 



