78 THE ORIGIN OF THE NERVOUS SYSTEM 



Moreover, if the original region of high rate persists 

 the electrical conditions and so the gradient as a whole 

 must persist and become static instead of advancing in 

 the protoplasm. The length of such a gradient will 

 depend, first, upon the potential arising at the original 

 region of rapid oxidation and, second, upon the con- 

 ductivity of the protoplasm. In that they represent a 

 gradation from a region of high to one of low physio- 

 logical activity, such gradients are comparable to the 

 ascending phase or front of an excitation wave in a nerve 

 fiber, viz., that portion of the wave from the region 

 where excitation is just beginning to the region of maxi- 

 mal excitation. But they differ from the ascending phase 

 of the nervous impulse in that they do not advance, though 

 they may undergo change in length with change in the 

 rate of reactions in the original region or at other levels. 



It seems probable that the most primitive excitation- 

 transmission phenomena in protoplasm possess more 

 nearly the characteristics or such oxidative or metabolic 

 gradients than those of the highly specialized sorts of 

 excitation and transmission processes, such as the nerve 

 impulse. Apparently all gradations exist between the 

 two extremes, and it is at least possible that the more 

 rapidly arising and more rapidly reversible excitation- 

 transmission processes, even in protoplasm in general, 

 are chiefly or wholly plasma-membrane processes, 

 while the slower, more persistent, and less readily revers- 

 ible processes approach more closely the oxidation 

 gradients and are not necessarily limited to cell mem- 

 branes, but may occur in relation to limiting surfaces 

 or phase boundaries more or less distant from the surface. 

 But since protoplasm is a system in which a considerable 



