ig 2 SCIENCE PROGRESS. 



modern histologists are indicated by black circles and 

 surrounding semicircles, and it is to be noticed how by 

 their disposition the figure gives expression to a common 

 hypothesis embracing the facts of Wallerian degeneration, 

 and the probability of the law of forward direction in the 

 nerve circuit in so far as it may conceivably depend upon a 

 valve action of cells and pericellular network. The data 

 that render that law highly probable are the well-known 

 facts that excitation of the central end of an afferent nerve, 

 and of the peripheral end of an efferent nerve, provokes 

 movement, while that of the peripheral end of a pure 

 afferent nerve, or of the central end of a pure efferent 

 nerve, produces no effect. 



I have alluded to the law of forward direction as being 

 "highly probable" only, inasmuch as there are experi- 

 mental data on record contradictory of that law, which 

 cannot therefore properly be presented as an undisputed 

 statement. Gotch and Horsley, while confirming the fact 

 that impulses up the efferent root cannot act retropulsively, 

 conclude that efferent or retropulsive impulses can pass 

 down afferent channels. Wundt again accepts the possi- 

 bility of retropulsive discharge along afferent tracts by 

 admitting that a hallucinatory disturbance of the brain may 

 be propagated down the optic nerve to the retina. Never- 

 theless, on a balance of probabilities, I am tempted to agree 

 with James, who regards the law as an indubitable truth 

 rather than as an hypothesis. 1 



BIBLIOGRAPHY. 



Wundt. Physiol. Psychol., vol. ii., p. 529 (4th ed.). 

 Gotch and Horsley. Phil. Trans. R. S., 1891. 

 SCHAFER. Brain, p. 167, 1893. 

 James. Psychology, vol. ii., p. 581. 



A. D. Waller. 



1 And Schafer, in his recent presidential address to the Neurological 

 Society, arrives at the same conclusion, viz., " that the ordinary centrifugal 

 paths are blocked for centripetal impulses, although the centripetal paths 

 may convey centrifugal impulses, this physiological difference being cor- 

 related with a difference of anatomical relationship at the junction of the 

 respective nerve-elements". 



