THE CIRCULATION OF THE BLOOD 533 



The increased intracardiac pressure acts as a 

 constant stimulus to the cardiac muscle and the 

 hitherto inactive apex begins to contract again. 



Thus a constant stimulus may discharge peri- 

 odic contractions in a muscle habituated to 

 periodic contractions (compare page 144) ; the 

 galvanic current and chemical stimuli, such as 

 delphinin, are further examples of constant stim- 

 uli which call forth rhythmic contractions of the 

 heart muscle. 



The Inactive Heart Muscle still Irritable. Stim- 

 ulate the inactive " apex " mechanically and with 

 single induction shocks. 



The apex, though incapable of spontaneous 

 rhythmic contractions, is still irritable, and will 

 respond by a single contraction to each stimulus. 



Refractory Period ; Extra-Contraction ; Compen- 

 satory Pause. Put the electromagnetic signal 

 in the primary circuit. Connect the binding 

 posts on the heart-holder to the secondary coil of 

 the inductorium. Arrange the latter for single 

 induction currents. Place the ventricle on the 

 heart-holder. Send maximal make and break 

 induction currents through the ventricle from 

 time to time in each phase of the cardiac cycle. 



Note that (1) the stimulus sometimes calls 

 forth an extra-contraction ; (2) at other times 

 the stimulus causes no contraction, having fallen 



