PROPERTIES OF THE HEART MUSCLE. 575 



exhibited by the crustacean (lobster) heart, but has been shown 

 to be true for the mammalian heart muscle.* 



2. The refractory period of the beat. It was shown by Marey f that 

 the heart muscle is irritable to artificial (electrical) stimuli only 

 during the period of diastole. During the period of systole an elec- 

 trical stimulus has no effect ; during the period of diastole such a 

 stimulus calls forth an extra systole and the latent period pre- 

 ceding the extra contraction is shorter the later the stimulus is 

 applied in the diastolic phase. This relationship is well shown by 

 Marey 's curves reproduced in Fig. 236. The period of inexcitabil- 

 ity is designated as the refractory period of the heart beat. Marey 

 defined this refractory period as falling within the first part of the 

 systole, and stated that its duration varies with the actual strength 

 of the stimulus. Later experiments by other investigators make it 

 probable that the refractory period lasts during practically the entire 

 systole. t 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, is not refractory during systole, 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. 567) 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. 



* For experiments on mammalian heart and literature, see Woodworth, 

 "American Journal of Physiology," 8, 213, 1903. 



f Marey, "Travaux du laboratoire," 1876, p. 73. 



j See paper by Woodworth, loc. cit. Also Schultz, "American Journal of 

 Physiology," 16, 483, 1906, and 22, 133, 1908. 



