PROPERTIES OF THE HEART MUSCLE. 525 



of the stimulus. Later experiments by other investigators make it 

 probable that the refractory period lasts during practically the entire 

 systole.* According to this point of view, therefore, the heart muscle 

 during its 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 cannot be thrown into complete 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 may throw some light also on the rhythmical character 

 of the beat. The occurrence of the refractory period and the 

 subsequent gradual return of irritability are connected 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 muscle 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. 517) 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, f 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 of exact compensation 

 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 stimulated so as to give an extra beat out of sequence it will remain 

 in diastole until the next auricular beat stimulates it, and will thus pick up 

 the regular sequence of the heart beat. 



'The Normal Sequence of the Heart Beat. The normal 

 rhythm of the heart beat is first a contraction of the auricles, then 

 one of the ventricles, or probably first the mouths of the great veins, 

 then auricle and ventricle. This sequence from venous to arterial 



* See paper by Wood worth, loc. cit. Also Schultz, "American Journal of 

 Physiology," 1906, xvi., 483. 



fCushny and Matthews, "Journal of Physiology," 21, 227, 1897. 



