PROPERTIES OF THE HEART MUSCLE. 523 



living substance constitutes or leads to the development of the 

 inner stimulus. 



Physiological Properties of Cardiac Muscle. Cardiac muscle 

 exhibits certain properties which distinguish it sharply from skeletal 

 muscular tissue and which have a direct bearing upon the rhyth- 

 micity of the contractions and the sequence shown by the different 

 chambers. The most characteristic of these properties are the 

 following : 



1. The contractions of heart muscle are always maximal. In 

 skeletal muscle and in plain muscle the extent of contraction is 

 related to the strength of the stimulus, and we recognize the exis- 

 tence of a series of submaximal contractions of varying heights. 

 This is not true of heart muscle. As was first shown by Bow- 

 ditch, a piece of ventricular muscle when stimulated responds, if 

 it responds at all, with a maximal contraction. The apex of a 

 frog's heart does not beat spontaneously, but contracts upon 

 electrical stimulation. If such an apex is connected with a lever 

 to register its contractions, and the electrical stimulus applied to 

 it is gradually increased, the first contraction to appear is maxi- 

 mal, and it is not further increased by augmenting the stimulus. 

 This property is sometimes described by saying (Ranvier) that 

 the contraction of the heart muscle is all or none. This fact 

 must not, however, be interpreted to mean that the force of 

 contraction of heart muscle is invariable under all conditions. 

 Such is not the case. The heart muscle under favorable nutritive 

 conditions may give a much larger and more forcible contraction 

 than is possible under conditions of poor nutrition; but the point 

 is, that, whatever may be the condition of the muscle at any 

 given moment, its contraction in response to artificial stimulation 

 is maximal for that condition, that is, does not vary with the 

 strength of the stimulus. As was said above, this property is not 

 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 contraction and the latent period 

 preceding 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. 216. The period of inexcitability 

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



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

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

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



