ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 149 



the whole of it, is retracted. The more distant pseudopodia, how- 

 ever, still remain unaffected, or retract only a very little, and that 

 gradually." Finally, with very strong stimulation, the process of 

 contraction may extend to all the pseudopodia, till the whole 

 mass is withdrawn. " The stimulated pseudopod is drawn back 

 most quickly, almost instantaneously, while the others follow 

 more slowly in proportion with their distance." 



It follows that stronger stimuli not only produce a quicker 

 reaction than weak stimuli, but that the effects are more widely 

 diffused, i.e. the effect diminishes with distance from the point of 

 excitation. 



Although it is a priori probable that the same is true of 

 conduction of every excitatory process in all living substance, 

 its direct proof is very difficult wherever excitability and 

 conductivity are highly developed, since the difference, owing 

 to the inconsiderable length of the tracts available, must be 

 minimal. This notwithstanding, Bernstein succeeded in demon- 

 strating that the wave of contraction in striated frog's muscle 

 undergoes a perceptible diminution (a " decrement ") during its 

 course, whence it follows that the expansion- curve of a directly 

 excited point of the muscle is invariably higher than that 

 of a more distant point excited with the same stimulus. It 

 must, however, be remembered that these experiments relate to 

 excised muscles, in which nutrition is no longer normal, so 

 that, as du Bois-Eeymond pointed out, the decrement observed 

 might well be a manifestation of the dying muscle. And, indeed, 

 we shall see from certain galvanometric effects in uninjured 

 muscle, to be discussed below, that a decrease in the excitation 

 wave preceding the wave of contraction is not perceptible. 



In view of the significant differences in velocity of the 

 contraction process in striated muscles of different animals, and 

 even in different muscles of the same species, it is not surprising 

 that similar differences should exist in regard to conductivity, the 

 muscular twitch being in general only the expression of a contrac- 

 tion spreading itself from the point of excitation over the entire 

 muscle. Accordingly the rapidity with which excitation, or con- 

 traction, is transmitted varies in the same instances and the same 

 sense as the curve of the twitch, so that its rapidity may be said to 

 vary directly with the magnitude of the latter. According to Her- 

 mann and Aeby the velocity in the tortoise averages G'5-1'8 m. ; 



