VOLUNTARY CONTRACTION 267 



causing a shortening and thickening of those elements ; and (2) the 

 further change of this substance into one which can escape by 

 diffusion, or into a substance with a low surface tension, so that 

 now the muscle relaxes and can be stretched by any extending 

 force. If these two phases went on continuously, but the first phase 

 kept ahead of the second one, a continuous state of contraction 

 would be produced in the muscle. Since the contraction of the 

 muscle only occurs in response to impulses from the central nervous 

 system, we should have to imagine also a continuous stream, e.g. of 

 negatively charged ions, descending the nerve and evoking an 

 excitatory change in the muscle-fibres as they impinge on the neuro- 

 muscular junction. We have evidence that a state of excitation 



FIG. 93. Continued contraction followed by rhythmic contractions of a 

 muscle in response to a constant stimulus. (BIEDERMANN.) 



The muscle was excited by the passage of a constant current, the 

 cathodal end having been moistened with a weak solution of Na COo. 



of a nerve, which is apparently continuous, may excite a corre- 

 spondingly continuous state of excitation in the muscle attached. 

 During the passage of a constant current through muscle there is 

 a continuous contraction in the neighbourhood of the cathode. 

 If the irritability of the muscle at this point be increased by 

 the application of a solution of sodium carbonate, Biedermann 

 has shown that this excitation is propagated to the rest of the 

 muscle, and on closure of the current we obtain a prolonged 

 contraction followed by rhythmic contractions (Fig. 93). More- 

 over in frogs, the excitability of which has been heightened by 

 keeping them at 2 to 3 C. for some days, the closure of a 

 descending current through the sciatic nerve causes a prolonged 

 contraction of the gastrocnemius ; and in the same way there may b.e 

 a prolonged contraction produced by the opening of an ascending 

 current through the nerve. 



The question however can only be decided by experiment. If 

 a voluntary or reflex contraction is of the nature of a tetanus, we 

 should be able, by a study of the mechanical and electrical phenomena 

 combining the contraction, to obtain distinct evidence of this 

 causation. It was shown by Wollaston that, on listening to a 

 contracting muscle, a low sound was heard, which, according to him, 



