2 80 THE GROWTH OF THE BRAIN. 



periments by Gotch and Horsley, which show that the 

 nerve impulses may pass out of the spinal cord by way 

 of the dorsal roots, although usually they are pictured 

 only as passing in. 1 On the other hand, these observers 

 found that if a ventral root were stimulated the nervous 

 impulse could not be detected as passing in any of the 

 columns of the cord ; apparently it was blocked in the 

 cell-bodies of these ventral elements. This fact does 

 not prove, however, that such an impulse is without 

 influence in modifying both the reactions and the 

 nutrition of the cell-body in which it seems to disappear. 



These beautiful researches of Gotch and Horsley have 

 shown that the strength of a nerve impulse as measured 

 in the columns of the spinal cord is much diminished 

 when it emerges through a ventral root. There may be 

 several causes for this, but the fact of interest is the 

 smallness of the electrical disturbance accompanying 

 the nerve impulse under these conditions. 



The electrical variation passes along the nerve in the 

 form of a wave, so that a simultaneous disturbance 

 may be detected extending over a piece of nerve some 

 1 8 mm. long, the crest of the wave being somewhat in 

 advance. The rate at which this wave travels has been 

 often studied, but without concordant results. The 

 determination of this rate in a frog's nerve was one of 

 the early triumphs of von Helmholtz, who found it to 

 be about 30 meters per second. In warm-blooded 

 animals the best observations indicate a rate of about 40 

 meters per second. Reducing these to expressions with 

 which we are more familiar, these rates are represented 

 by that of a railroad train running at 68-90 miles an 

 hour, or a horse going at the rate of one mile in 40-54 

 seconds. These figures should be applied with caution, 

 since they are the averages of widely divergent single 

 1 Gotch and Horsley, Proc. Roy. Soc., London, 1888. 



