TRANSMISSION AND CONDUCTION 211 



type "neuroid" transmission. While this term is 

 undoubtedly useful, it should not be forgotten that 

 such neuroid transmission represents merely a stage in 

 the development and evolution of the excitation- 

 transmission process and its morphological substratum. 

 Even though a definite, anatomical conducting path 

 is not distinguishable, the direction of transmission of 

 the impulses in ciliated surfaces is more or less definitely 

 determined. Griitzner (1882) and Kraft (1890) observed 

 that the increased ciliary activity following direct 

 mechanical stimulation of a local area of a ciliated 

 epithelium is transmitted very largely in one direction 

 from the stimulated area, the effect being observed as a 

 band of increased activity, becoming somewhat broader 

 with increase in distance from the stimulated area, but 

 remaining definite in direction. Briicke's reimplanta- 

 tion experiment, referred to above, also shows that the 

 direction of transmission is definite. As far as I am 

 aware, no adequate physiological grounds for such 

 definiteness of direction in the absence of a definite 

 anatomical path have been discovered by earlier authors, 

 but it is evident from what has been said in earlier 

 chapters that the physiological gradients afford a basis 

 for definitely directed transmission, even in the absence 

 of localized and directed anatomical paths. We expect 

 transmission to occur chiefly downward in such gradient, 

 and, as a matter of fact, in those cases in which the 

 direction of the physiological gradients is known this is 

 very generally, if not always, the case as regards the 

 usual or normal direction. In many ciliated surfaces 

 the direction of transmission may be reversed by ade- 

 quate stimulation of lower levels of the gradient, but 



