132 A TEXTBOOK OF PHYSIOLOGY 



make the same number of beats as usual in a given time, in spite 

 of the induction of extra systoles. The length of the compensatory 

 pause is due to the refractory period ; the impulse causing the normal 

 contraction reaches the ventricle at a time when it is in a state of 

 systole induced by the artificial stimulus. It is therefore refractory, 

 and the normal impulse has no effect. The ventricle is not stimu- 

 lated again until the next normal impulse arrives, and thus the pause 

 is produced. 



In the case of the frog's heart, a compensatory pause does not 

 follow extra contractions induced by stimulations of the sinus. Extra 

 systoles, followed by the compensatory pause, are produced by a 

 stimulus applied to some other part of the heart e.g., auricle or 

 ventricle. It is assumed, therefore, that the normal stimulus actually 

 arises in the sinus. 



This is also true for the mammalian heart. Electrical stimulation 

 of the sinu-auricular node does not induce an irregularity of rhythm, 

 but either a slowing or a weakening of the whole heart, according to 

 the intensity of the stimulus. 



FIG. 41. NORMAL ELECTRO-CARDIOGRAM. (W. T. Ritchie, from Cowan's "Diseases 



of the Heart.") 



Normal cardiac muscle cannot be tetanized (Fig. 40), as it is 

 impossible to bring about a true summation of stimuli in the normal 

 heart. In Fig. 38 it is seen that a second stimulus applied towards 

 the end of systole can only produce a small amount of summation, 

 because it does not act sufficiently soon after the first stimulus owing 

 to the refractory period. 



The cardiac muscles of certain invertebrates for example, of the 

 horseshoe crab, Limulus do not possess the characteristic properties 

 of the vertebrate heart-tissue. It possesses no refractory period, 

 gives submaximal contractions, and can be tetanized. 



Like other forms of muscle, the heart-muscle possesses the property 

 of tonicity. The heart may be tonically contracted or dilated, and 

 by its systole expel the blood from a larger or a smaller cavity. The 

 mammalian heart can be placed in an instrument called the " cardio- 

 meter" (Fig. G8), and its volume recorded and the alterations of 

 tonicity measured. The tonicity is influenced by the cardiac nerves. 



If the frog's ventricle be placed in a weak solution of caustic soda 

 (1 in 20,000 of normal saline), it relaxes less and less between the beats, 



