240 PHYSIOLOGY 



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 excita- 

 tion of a nerve, which is apparently continuous, may excite a correspondingly 

 continuous state of excitation in the muscle attached. During the passage 

 of a constant current through muscle there is a continuous contraction in 



FIG. 92. Continued contraction followed by rhythmic contractions of a muscle 



in response to a constant stimulas. (BIEDEBMENN.) 



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

 end having been moistened with a weak solution of NaC03. 



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. 92). Moreover in frogs, the ex- 

 citability 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 be a prolonged contraction produced by the opening of an ascend- 

 ing current through the nerve. 



The question, however, can only be decided by experiment. If a volun- 

 tary 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, corresponded to a vibration frequency of 

 36 to 40 per second. The same observation was made by Helmholtz, and 

 can be repeated by any one who will place the end of a stethoscope on a 

 muscle, e.g. the biceps, and listen to the sound produced when it contracts. 

 Helmholtz pointed out, however, that the tone heard corresponded to the 

 resonance tone of the external ear, and was the same as that noted when 

 listening to any irregular sound of low intensity. Thus the roar of London 

 that we hear in the middle of Hyde Park has the same pitch as the muscle 

 sound of the contracting biceps. The muscle sound, therefore, teaches us 

 nothing as to the pitch or number of contractions per second making up the 



