158 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



In some cases, the extra stimulus provokes not merely one, but two or three 

 extra contractions. 



The amplitude of the extra contraction increases with the length of the 

 interval between the maximum of contraction and the extra stimulus. If the 

 extra stimulus is given at the beginning of relaxation, the extra contraction is 

 exceedingly small ; on the other hand, the extra contraction may be greater 

 than the primary one, when the stimulus falls in the pause between two normal 

 beats. 



The supplementary systole of the auricle is sometimes followed by a sup- 

 plementary systole and compensatory pause of the ventricle, sometimes by the 

 compensatory pause alone, probably because the excitation wave reaches the 

 ventricle during its refractory period. Multiple extra contractions of the 

 auricle are often followed by the same number of extra contractions of the 

 ventricle. If the frog's heart is made to beat in reversed order, ventricle 

 first, auricle second, extra contractions of the ventricle may be produced, and 

 will cause extra contractions of the auricle with compensatory pause. If the 

 reversed excitation wave travelling from the ventricle to the auricle reaches 

 the latter during auricular systole, the extra auricular contraction is omitted, 

 but a distinct though shortened compensatory pause is still observed. The 

 phenomena with reversed contraction are therefore similar to those seen under 

 the usual conditions. 1 



Kaiser finds in frogs poisoned with muscarin that stimulation of the ven- 

 tricle during the refractory period causes the contraction in which the stimulus 

 falls to be more complete, as shown by the contraction curve rising above its 

 former level. He concludes that the ventricle is not wholly inexcitable even 

 during the refractory period. 



The question whether the refractory state and compensatory pause are 

 properties of the muscle-substance or of the nervous system of the heart has 

 excited considerable attention. If the ganglion-free apex of the frog's ven- 

 tricle is stimulated by rapidly repeated induction shocks it can be made to con- 

 tract periodically for a time. By momentarily increasing the strength of any 

 one induction shock an extra stimulus can be given from time to time. When 

 the extra stimulus falls after the contraction maximum or during diastole an 

 extra contraction results, otherwise not. The refractory period exists, there- 

 fore, independently of the cardiac ganglia. 



The compensatory pause can also, though not always, be secured with the 

 ganglion-free apex. 2 



The refractory period has been used to show how a continuous stimulus 

 might produce a rhythmic heart-beat. The continuous stimulus cannot affect 

 the heart during tin; refractory period from the beginning to near the maxi- 

 mum of systole. At the close of the refractory period the constant stimulus 



1 Kaiser : Zeitschrift fur Biologic, L895, xxxii. p. 19. 



-Kaiser: Ibid., p. 449; for experiments on the embryo, see Pickering : Journal of Physi- 

 ology, 1896, xx. j>. 165. 



