148 COMPARATIVE PHYSIOLOGY 



of this type occurs when the first stimulus sets up a concentra- 

 tion of ions at the point of excitation which does not subside 

 till the second comes into operation. Summation of this kind 

 can obviously take place only when the nerve is stimulated in 

 both cases at precisely the same point. But the phenomena 

 of the refractory and supernormal states in muscle and nerve 

 allow for at least two other types of summation. These 

 summations can occur when the electrodes for the successive 

 stimuli are not at the same point : that is, w^hen the first stimulus 

 initiates a disturbance in the nerve itself but does not lead to 

 response in the peripheral organ. This may occur, either 

 because the second stimulus fell in the supernormal phase 

 of excitability of the peripheral tissue ; or because the second 

 impulse falls in the period in which the nerve itself conducts 

 with supernormal intensity, so that if the intensity of the 

 first impulse were just insufficient to penetrate the junction 

 of nerve and muscle the second may succeed. 



An example of the latter is provided by the work of Richet, 

 Lapicque, and Keith Lucas upon the crayfish claw. The 

 phenomena in this case are complicated, it may be remarked, 

 by the existence of a double neuromuscular mechanism ; one 

 concerned with sustained contraction, and the other with 

 rapid closing of the pincers. This is shown by the fact that 

 a diflFerent type of response is given by strong currents of short 

 duration (twitch-like movement) from that elicited by weak 

 currents of long duration (protracted closure). A similar 

 state of aflFairs was shown by Keith Lucas to exist in the 

 lobster, Homarus, where the curve relating duration of con- 

 denser discharge to the minimum potential requisite to pro- 

 duce contraction of the adductor showed a discontinuity on 

 one side, of which the response was a rapid twitch and on the 

 other a sustained movement. Lapicque showed a somewhat 

 similar phenomenon in the claw of Carcinus. Richet showed 

 that when the abductor nerve of the claw in Astacus is initially 

 stimulated a small twitch results. If this is followed by two 

 shocks delivered in rapid succession a more powerful con- 

 traction results. If the preparation is stimulated every half- 

 minute alternately by a single shock and by two successive 



