3 io 



TEXT-BOOK OF PHYSIOLOGY 



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. 138). 



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 

 of the diastole (Fig. 139). For this 

 reason weak stimuli are more effective 

 in the later than in the earlier period 

 of the relaxation and the diastole. 



After the development and disap- 

 pearance of the extra systole a consider- 

 able pause in the heart's action occurs 

 to which the term compensatory pause 

 has been given on the assumption that 

 it was necessary on the part of the 

 heart to compensate for the disturbance 

 of the rhythm by remaining at rest until the time of the next beat arid 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- 

 sults of the same methods of stimulation are different. Though an extra 



FIG. 138. 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 

 line e, indicates the time the stimulus is 

 sent in. In i, 2, 3, the stimulus falls into 

 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 

 period or the period of returning and in- 

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

 systole and during the diastole. The width 

 of the shaded lines is a measure of the 

 latent period, i.e., the period between the 

 time of stimulation and the beginning of 

 the resulting contraction which diminishes 

 as the diastole progresses. 



