CONDUCTION WITHIN NERVE CELLS. 



605 



ascertain whether there is any delay in cases where the nerve impulse is 

 compelled to pass through the body of the nerve cell, as it must do, for 

 example, in bipolar cells ; this is a point which has yet to be determined. 



The only chemical change which has been noticed in the bodies of 

 nerve cells, as the result of activity, is a change in the direction of 

 acidity, due apparently to the formation of lactic acid, as in muscle. 

 That the fatigue products of the nerve centres and of muscles are 

 similar, is shown, first, by the fact that the muscles may give evident 

 signs of fatigue, merely as the result of prolonged nervous (mental) 

 activity ; x and, secondly, by the evident fact that muscular fatigue is 

 detrimental to nervous (intellectual) activity in general, and not only to 

 the particular parts of the central nervous system which come into 

 action during the muscular effort. In nerve fibres the only change 

 which is apparent as the result of excitation is the change of electric 

 potential which accompanies the passage of the nervous impulse. It 

 might be expected that when the nerve centres are set in activity, the 

 cells in which the impulses are started would also exhibit changes of 

 potential. But, probable as this is, the demonstration of the fact is 

 beset with serious difficulties ; and, in spite of attempts which have from 

 time to time been made to overcome these difficulties, our knowledge on 

 this subject has remained in a very fragmentary and indecisive condition. 2 



The question whether the " sense of effort," which is closely 

 associated with the " sense of fatigue," is due to changes originating in 

 the nerve cells, or to sensory impressions passing to the nerve centres 

 from the contracting muscles, has been much debated. 3 The probability 

 is, that its origin is from both sources, and that the effects are 

 accentuated, with the supervention of fatigue, by the accumulation 

 of waste products of both nervous and muscular metabolism in the 

 circulatory fluid. 



Conduction within and between nerve cells. — It was conjectured 

 by Golgi that the dendrons of a nerve cell were not for the conduction 

 of nerve impulses, but might merely serve the purpose of rootlets, and 

 thus aid to obtain pabulum for the nutrition of the cell. This con- 

 jecture has, however, been abandoned, since it has been discovered 

 that in many cases nervous impulses, in passing along a nerve cell, 

 must of necessity traverse one or more dendrons as well as the 

 axon. Granting, however, that both dendrons and axons serve as 

 nerve conductors, the question suggests itself, Is there any specific 

 difference in the manner or direction by which the two kinds of pro- 

 cesses conduct impulses ? 



van Gehuchten 4 and Kamon y Cajal 5 have attempted to lay down, 

 as a general law of conduction in the nerve cell, the proposition that all 

 nervous impulses within the cell are axipetal, that is, that they all pass 



the same path through the cord to produce the reflex movement which was used as a 

 measure of the relative time occupied. 



1 Mosso, "Die Ermiidung"; Arch. f. Physiol., Leipzig, 1890, S. 129. 



2 See on this subject R. Caton, Pep. Brit. Ass. Adv. Sc, London, 1875 ; Beck, Gcntralbl. 

 f. Physiol., Leipzig u. Wien, 1890, Bd. iv. ; Fleischl von Marxow, ibid.; Danilewsky, ibid. , 

 1891, Bd. v. ; Beck and Cybulski, ibid., 1892, Bd. vi. 



3 For discussion of this and cognate points in neural physiology, see Waller, " On 

 the Sense of Effort," Brain, London, 1891, vol. xiv. p. 179; and "On the Functional 

 Attributes of the Cerebral Cortex," ibid., 1892, vol. xv. p. 329. Cf. also Bastian, "On 

 the Neural Process underlying Attention and Volition," ibid., 1892, p. 1. 



4 "Systeme nerveux," 1897. 



5 Rev. trim, de histol., 1897. 



