300 PHYSIOLOGY OF 2ICSCLES AND NERVES. 



together effect a greater pulsation tlian does a single irrita- 

 tion. 



If more than two irritants follow each other in rapid 

 succession, tetanus results, as we know. In this case also 

 the height of elevation is always greater than that which 

 can be attained by a single pulsation. For the muscle has 

 the power of being again irritated even when it is already in 

 the act of contraction, a more powerful contraction being 

 thus induced in it. The bearing of these facts on the case 

 of nerve is that the separate excitements effected in it by 

 these rapidly successive irritations do not mutually disturb 

 each other, but are transmitted one after the other, in the 

 sequence in w^hich they originate, to the muscle on which 

 they act. But when the number of the irritants becomes 

 too great, the nerve-molecules are no longer able to keep 

 pace -with the rapidly succeeding shocks, and the nerve is 

 unexcited. The limit at which this intervenes has, how- 

 ever, not yet been determined with any certainty. It 

 appears to lie at between SOO to 1000 irritants per second. 



4. Curve of Excitability. Eesistaxce to Transmission 



(p. 123). 



The increased excitability at the upper parts of the un- 

 injured sciatic nerve, when not severed from the body, 

 which, on the strength of our earlier experiments, we have 

 assumed in the text, has recently been again defended by 

 Tiegel against various objections. For reasons explained in 

 the text it is inadmissible to infer an avalanche-like increase 

 in the irritation merely from this higher excitability 'of the 

 upper parts. Beside the experiments of Munk alluded to 

 on page 116, there are other experiments from which a 

 resistance to transmission in the nerve may be inferred. 

 Such a resistance, weakening the irritant during its propa- 

 gation, and an avalanche-like increase in the irritant, are 

 irreconcilable contradictions which mutually exclude each 



