PROPERTIES OF THE HEART MUSCLE. 569 



parent and due to the fact that in the former the fibers are separated 

 from one another by the sarcolemmal sheaths. They hold that in 

 skeletal as well as in heart muscle the contraction of each fiber is 

 all or none, and that so-called submaximal contractions are simplj^ 

 contractions in which less than the whole number of fibers partici- 

 pate. 



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. 237a. The period of inexcitabil- 

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

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

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

 of the stimulus. Later experiments bv other investigators make it 

 probable that a refractory period lasts during practically the entire 

 systole, t In the first part of the systole the refractory condition 

 is absolute, that is, a response cannot be obtained with any 

 stimulus. In the latter part of the systole the refractory condi- 

 tion is relative, strong stimuli may provoke a contraction. Ac- 

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

 its period of actual contraction is unirritable, and in this respect 

 it offers a striking difference to skeletal and plain muscle. The 

 existence of this long 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 irritabihty, 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 irritabihty during 

 the diastole may throw some light also on the rhythmical character 

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

 sequent 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 



* Marey, "Travaux du laboratoire," 1876, p. 73. • 



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

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



