150 PRACTICAL PHYSIOLOGY 



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. The periods of contraction and relaxation 

 will together last 2 sec. The contraction is much slower than that 

 of striated muscle. 



FIG. 151,-Stanniused heart. Staircase effect produced by excitations at the 

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



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

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

 at a 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. 1 50). 



Staircase Phenomenon. A Stannius preparation is 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 con- 

 traction. The height of the contraction depends on the condition of 

 the heart muscle, not on the strength of the stimulus, so long as the 

 latter is effective. For the first few beats each contraction makes the 

 heart more excitable. The same phenomenon is observed in the muscle 



