ADVANCED EXPERIMENTAL PHYSIOLOGY 339 



drum. Stop the drum immediately after recording the contraction. 

 Close the short circuiting key, then close the trigger key; lastly 

 open the short circuiting key. Bring the drum round slowly by 

 hand until the striker just opens the trigger key. The heart will 

 contract and the lever write a line marking the moment of excitation. 

 Take another curve with the electrodes placed on either side of the base 

 of the ventricle. The latent period will be less. In the first case the 

 excitatory wave was delayed in the auriculo-ventricular groove. With 

 the tuning fork (100 per sec.) take a time tracing just beneath the 

 heart curves, and measure the latent period. It equals about 0*1 sec, 



FIG. 218. Staimiused heart. Staircase effect produced by excitations at the points 

 marked on the lowest line. The time is marked in seconds. (L.H.) 



The periods of contraction and relaxation will together last 2 sec. 

 The contraction is much slower than that of striated muscle. 



Any Stimulus, if effective, causes a Maximal Contraction. Place a 

 spring key and an electric signal in the primary circuit. Set the drum 

 at the slow rate, and bring the heart lever and signal to write on the 

 drum. Record the effect of excitation at intervals of a minute or 

 more, with varying strengths of current. The heart gives 'all or 

 nothing/ i.e. if excited at all it gives its full contraction. 



The Refractory Period. Record the effect of throwing in a second 

 excitation (a) during the systole, (b) during diastole. The heart is 

 refractory during the whole period of systole, i.e. it makes no response 

 to a second stimulus. The excitability returns with diastole, and 

 becomes greater as diastole proceeds. (Fig. 217). 



Staircase Phenomenon. A Stannius preparation is placed under the 

 lever (Fig. 59), and excited with single induction shocks once in every 

 five seconds. The stationary drum is moved on by 2 mm. between the 

 excitations. The heights of the first four or five contractions form an 

 ascending series. The heart responds to any stimulus which is effective 

 by a maximal contraction. The height of the contraction depends on 

 the condition of the heart muscle, not on the strength of the stimulus, 



