288 TEXT-BOOK OF PHYSIOLOGY. 



JB^ heart-cells ; that the potassium chlorid acts antagonistically to the 

 calcium.! From these facts it may be inferred that the stimulus is 

 chemic in character and, though continuously acting, calls forth but 

 periodic contractions. 



PROPERTIES OF THE HEART-MUSCLE. 



1. Irritability. The heart-muscle in common with other muscles 



possesses irritability in virtue of which it responds by a change 

 of form to the action of a stimulus. Whatever the stimulus, 

 here, as elsewhere, there .is a conversion of potential into kinetic 

 energy heat and mechanic motion. The normal physiologic 

 stimulus is at present undetermined. - In common with other 

 forms of muscle tissue, the heart may be made to contract' by 

 artificial stimuli e. g., mechanic, thermic, chemic, and electric. 

 The irritability depends on the nutrition, and so long as this is 

 ' maintained the muscle will respond by a contraction to any stim- 

 ulus. The irritability is most marked in the neighborhood of the 

 venae cavae terminations. It is least marked in the ventricles. 



2. Conductivity. The heart-muscle possesses conductivity. The 



excitation process and the subsequent contraction wave, both of 

 which take their rise under physiologic conditions near the venas 

 cavse terminations, are conducted over the auricles, thence to the 

 ventricles from base to apex. The propagation of both processes 

 is accomplished by muscle-tissue alone, independently of the 

 nerve systems. The conductivity, however, is not equally well 

 developed in every part of the heart. This is especially true of 



the tissue at the auriculo-ventricular 

 junction... At this point the contrac- 

 tion wave is delayed for an appreci- 

 able period, a condition due to the 

 embryonic character of the muscle- 

 tissue. In the frog's heart the excita- 

 tion process begins in the sinus ven- 



FIG. 125. RECORD OF THE osus, from which it passes to the 

 F THE auricles > thence to the ventricles. The 

 excitation process as well as the con- 

 traction wave is delayed both at the 



smu-auricular and auriculo-ventricular junctions. In Fig. 125, 

 which is a graphic record of the heart-beat, the two elevations 

 of the lever on the up-stroke, a and b, represent the contraction 

 of the sinus and the auricle respectively, while the two depres- 

 sions c and d indicate the delay in the transmission of the con- 

 traction wave at the two junctions. There is here an anatomic 

 obstacle to the conduction of the contraction. This may be 

 artificially increased by compressing the heart between the 



