506 CIRCULATION OF BLOOD AND LYMPH. 



of the stimulus. Later experiments by other investigators make 

 it probable that the refractory period lasts during the entire systole.* 

 According to this point of view, therefore, the heart muscle during 

 its whole period of actual contraction is entirely unirritable, and in 

 this respect it offers a striking difference to skeletal and plain muscle. 

 The existence of this refractory period explains why the heart 

 muscle can not be thrown into tetanic contractions by rapidly 

 repeated stimuli. Since each contraction is accompanied by a 

 condition of loss of irritability, it is obvious that those stimuli that 

 fall into the heart during this period must prove ineffective. The 

 refractory period and the gradual increase in irritability during the 

 diastole throw some light also on the rhythmical character of the 

 beat. Assuming that the inner stimulus is a constantly acting 

 stimulus, its effect must necessarily be to produce rhythmical 

 contractions if the heart muscle at each contraction falls into a 

 condition of non-excitability from which it recovers only gradually. 

 On the other hand, if the production of the inner stimulus is rhyth- 

 mical, as is suggested in the preceding paragraph, the relatively slow 

 development of irritability after a contraction must influence the 

 actual rhythm of the heart beat. The occurrence of the refractory 

 period and the subsequent gradual return of irritability are con- 

 nected no doubt with the metabolic changes taking place in the 

 heart muscle. It is in the character of this metabolism that we 

 must seek for the final explanation of these two phenomena and the 

 cause of the rhythmicity of the contractions. As was stated above, 

 it has been shown that the crustacean (lobster) heart does not obey 

 the all-or-none law, shows no refractory period, and is capable of 

 giving tetanic contractions when rapidly stimulated. In all these 

 respects it differs from the typical heart muscle of the vertebrate, 

 but the difference is perhaps sufficiently explained by the discovery 

 (p. 498) that the crustacean heart, in one form at least, is not an 

 automatically rhythmical tissue. Its rhythmical contractions, like 

 those of the diaphragmatic muscle in the higher vertebrates, depend 

 upon rhythmical impulses received from nerve centers. 



The Compensatory Pause. It has been observed that when an extra 

 systole is produced by stimulating a ventricle it is followed by a pause longer 

 than usual; the pause, in fact, is of such a length as to compensate exactly 

 for the extra beat ; so that the total rate of beat remains the same. The pro- 

 longed pause under these conditions is therefore frequently designated as the 

 compensatory pause. It has been shown, t however, that the exact compen- 

 sation in this case is not referable to a property of heart muscle, but is due to 

 the dependence of the ventricular upon the auricular beat. When the auricle 

 or ventricle is isolated and stimulated the phenomenon is not observed. In 

 an entire heart, on the contrary, the beat originates at the venous end of the 

 auricle and is propagated to the ventricle. If the latter chamber is stimu- 



* See paper by Wood worth, loc. cit. 



t Cushny and Matthews, "Journal of Physiology," 21, 227, 1897. 



