208 INTRODUCTION TO GENERAL PHYSIOLOGY 



will be all of the same height if the stimulus is strong enough to 

 stimulate at all. This is because the whole of the heart muscle 

 contracts at each beat, all its cells being in functional connection 

 with each other, contrary to the case of the voluntary muscle, where 

 a varying number of fibres can be set into activity. 



The Refractory Period. Using the weakest stimulus found in 

 the last experiment to be effective, first produce a beat, and at 

 various stages in its course, judged by the eye, apply a second 

 stimulus. This will be found to have no effect until a certain stage 

 in the relaxation period has been reached. 



If the laboratory is warm, the contraction may be so rapid that 

 the experiment is difficult. If so, lumps of ice may be placed on 

 the cork around the heart. The water from the melting of the 

 ice must be prevented from reaching the heart by a little wall of 

 plasticine. 



In case the heart has recommenced beating by the time this 

 last experiment is arrived at, the first stimulation of the preceding 

 method is omitted, using instead of the artificial contraction, the 

 natural one, and applying the second stimulus at various points of 

 the natural beat, which may be slowed by ice as before. 



