176 THE CIRCULATION OF THE BLOOD 



cold-blooded animals is furnished by observing the effects of local heat- 

 ing or cooling of the various parts of the heart. In all rhythmically 

 acting structures it is well-known that heat increases the rate of the 

 rhythm and cold depresses it. If we locally warm the region of the 

 sinus, as by holding a heated wire near it the whole heart will immedi- 

 ately beat quicker; but if we locally heat the tip of the ventricle, no 

 alteration of rhythm will be observed to occur (Fig. 42). 



The establishment of the fact that the sinus dominates the heartbeat 

 that it is the pacemaker of the beat raises the question as to how the 

 impulse originated at this place is transmitted over the rest of the 

 heart, and here again a neurogenic and a myogenic hypothesis have to 

 be considered. Before going into this question, however, it will be well 

 for us to consider briefly the manner of response of cardiac muscle 

 fiber to a stimulus, because the behavior of cardiac muscle under such 

 conditions is considerably different in many regards from that of skel- 

 etal muscle, and it is to these differences that many of the peculiar 

 alterations in the beat observed after interfering with the conducting 

 structures between the sinus and the rest of the heart, are to be ex- 

 plained. 



The Physiological Characteristics of Cardiac Muscle 



It is necessary to bring the heart into a quiescent state in order to 

 investigate the properties of its musculature. This is accomplished by 



Fig. 43. Frog heart showing the position of the first and second ligatures of Stannius 

 (Hedon): /, auricles; 2, sinus; j. ventricle. It is the first ligature which brings the heart 

 to a standstill. 



the application of the Stannius ligature between the sinus and the auri- 

 cles (Fig. 43). After tightening the ligature the auricles and ventricles 

 become quiescent, and by observing the effects following the applica- 

 tion of electric or other stimuli we can compare the behavior of the 

 cardiac muscle with that of skeletal mttscle similarly stimulated. This 

 comparison is made because of the assistance which it offers in compre- 

 hending the properties of cardiac muscle. As a matter of fact, recent 

 investigations have shown that the differences between the two types of 

 muscle are not fundamental, since under certain conditions the one may 



