2i8 SENESCENCE AND REJUVENESCENCE 



among both plants and animals, and for excitations transmitted 

 through the general protoplasm, as well as those transmitted 

 through muscle and nerve. 1 In some of the lower animals the 

 gradual fading out, with increasing distance from the point of 

 origin, of the muscular contractions following a slight local stimu- 

 lation, affords a visible demonstration of the decrease in effective- 

 ness with transmission, and the relation between the distance from 

 the point of stimulation at which the contraction ceases to occur 

 and the strength of stimulation indicates further that the more 

 intense excitation is transmitted to a greater distance than the less 

 intense. And, finally, there can be no doubt that impulses may be 

 transmitted to greater distances over specialized conducting paths, 

 of which nerves are the most highly developed form, than through 

 the general protoplasm, and apparently some nerves conduct with 

 less decrement per unit of distance than others. 



Certain physiologists maintain that the medullated nerves of 

 vertebrates conduct impulses without any decrement. If this is 

 true, an impulse might be transmitted in such a nerve to an in- 

 finite distance from its point of origin. There are, however, certain 

 facts which indicate that even in these nerves a decrement does 

 occur in the course of transmission, although it is often so slight as 

 to be inappreciable under ordinary conditions in the relatively short 

 pieces of nerves usually available for experiment. In the first place, 

 the electrical change, the negative variation accompanying the 

 passage of a nerve impulse, has been shown to undergo decrease 

 with increasing distance from the point of stimulation, and the 

 effectiveness of the impulse in producing muscular contraction 

 decreases in the same way. Moreover, various investigators have 

 recorded the existence of a decrement in the intensity of the impulse 

 in partially anaesthetized nerves, and there is no reason to believe 

 that the partial anaesthesia alters the fundamental nature of the 

 nerve as conductor: in all probability it merely makes the nerve a 

 less efficient conductor, so that the decrement becomes apparent 



1 For general consideration of the whole subject of conduction see Fitting, '07, 

 for plants, especially pp. 91-93 and 122-24; Biedermann, '03, especially pp. 204-8; 

 and Verworn, '13, chap. vi. for animals. See also Boruttau, '01; Ducceschi, '01; 

 A. Fischer, 'i i ; Kretzschmar, '04; Lodholz. '13. 



