THE CIRCULATION OF THE BLOOD 



during the systolic period. This non-responsiveness of the heart may be 

 shown by throwing into it a second stimulus at any moment during the systole. 

 Whatever the moment or whatever the strength of the stimulus may be the 

 extent of the contraction remains the same. During the systolic period the 

 heart is said, therefore, to be refractory or non-responsive to a second 

 stimulus. If, however, a second stimulus of average strength be thrown 

 into the ventricle at any moment during the relaxation, a second contraction 

 will be developed, which is known as 

 the extra systole (Fig. 135). 



The Extra Systole. The extent 

 of this extra systole will be propor- 

 tional to the time at which the stimu- 

 lus is thrown into the ventricle as it 

 passes from the beginning to the end of 

 its relaxation. Whatever the extent of 

 the extra systole, its height is no 

 greater than that of the first systole. 

 For this reason it is believed a tetanic 

 contraction cannot be developed. If 

 the stimulus be thrown into the heart 

 just as the relaxation is completed, the 

 extra systole attains the same height as 

 the preceding systole. In passing from 

 the beginning to the end of the relaxa- 

 tion and into the diastolic or resting 

 period, it has been found that the 

 extra systole can be evoked by a stimu- 

 lus which is steadily decreased in in- 

 tensity It is evident from this fact 

 that the restoration of the energy- 

 yielding material and the return of the 

 irritability gradually increases from the 



beginning of the relaxation to the end line e, indicates the time the stimulus is 



of the diastole (Fig. 136). For this in - ^ i, 2, 3, the stimulus falls into 



reason weak stimuli are more effective 

 in the later than in the earlier period 

 of the relaxation and the diastole. 



FIG. 135. MYOGRAMS OF THE FROG'S 

 VENTRICLE SHOWING THE EFFECTS OF AN 

 INDUCED ELECTRIC CURRENT SENT INTO 

 THE VENTRICLE AT DIFFERENT TIMES OF 

 THE CYCLE. (Marey.) 



oo' indicates the beginning of the con- 

 traction in each series. The break in the 



creasing irritability, i.e., at the end of the 



the ventricle in the non-responsive period 

 or the refractory period, i.e., during the sys- 

 tole when the irritability has practically 

 disappeared. From 4 to 8 the stimulus 

 falls into the ventricle in the responsive 

 After the development and disap- period or the period of returning and in- 



pearance of the extra systole a consider- 



able pause in the heart's action occurs 



to which the term compensatory pause latent period, i.e., the period between the 



has been given on the assumption that Jjf 16 of stimulation and the beginning of 



the resulting contraction which diminishes 



it was necessary on the part of the a s the diastole progresses. 

 heart to compensate for the disturbance 



of the rhythm by remaining at rest until the time of the next beat and thus 

 restore the rhythm. This was thought to be a special property of the heart- 

 muscle. This view, however, is no longer entertained. For if an isolated 

 ventricle of a frog heart be employed and made to contract rhythmically by 

 an artificial stimulus, or if a spontaneously beating portion of the dog's 

 heart be employed for experimentation instead of the whole heart, the re- 



