THE HEART BEAT. 497 



animal the successive contractions of the veins, auricles, and ventricles may 

 be followed easily with the eye. According to Fredericq * the sequence is differ- 

 ent in the mammalian heart ; the cardiac beat begins in the right auricle at 

 a point lying between the mouths of the two cavse, and thence spreads rapidly 

 over the auricles and may penetrate for some distance over the veins con- 

 nected with the auricles. Whether or not this explanation of the sequence of 

 beat in the mammalian heart is correct will have to be determined by experi- 

 ment. It is not a fact that can be settled by mere ocular inspection. 



The continuity of the muscular tissue enables us to understand 

 how the contraction passes quickly from cell to cell in the direction 

 of the muscular fibers. In the mammalian heart when exposed to 

 view it is evident that the auricular systole is not sufficient to 

 empty its cavity, so far at least as the atrium is concerned. The 

 contraction of the auricular appendages is more forcible. The 

 contraction may be regarded as a rapid peristalsis which sweeps a 

 portion of the blood before it into the ventricle. The force of the 

 contraction has been determined in a number of cases. For the 

 auricle of the dog's heart it may be valued at 20 mms. Hg. The 

 systole of the ventricle is to the eye a simultaneous contraction of 

 the whole musculature. Various observers, however, have shown 

 that the wave of contraction travels over the heart with a certain 

 velocity, which for the human heart has been estimated at 5 m. 

 per second ( Waller). f It is probable that this wave starts at 

 the base of the ventricle and travels along the course of the fibers, 

 that is, first toward the apex and then into the interior of the heart, 

 ending in the papillary muscles. In fact, Roy and Adams have 

 demonstrated graphically that the contraction of the papillary 

 muscles occurs somewhat later than that of the wall of the ventricle. 

 The slight pause between auricular and ventricular systole may be 

 referred to the fact that the muscular bridge between the two 

 chambers is small. We have experimental evidence that the con- 

 traction wave proceeds more slowly through a narrow bridge of this 

 sort. In the dog's heart the time interval between the contractions 

 of the auricle and ventricle is about y 1 ^- second. Since the con- 

 necting auriculo-ventricular bundle has a length of 10 to 15 mms. 

 the velocity of the conduction through this bundle must be about 

 10 to 15 cms. per second. 



The Electrical Variation. The contraction of the heart 

 muscle, like that of skeletal muscle, is accompanied by an electrical 

 change. That is, where the muscle substance is in contraction its 

 electrical potential is different from that of the resting muscle. 

 The advancing wave of contraction causes a corresponding electrical 

 change. If two points of the heart are connected with an electrom- 

 eter an electrical current will be shown, since the electrical change 



* Fredericq, "Archives Internationales de Physiologic," 1906, iv., 57. 

 fSee Tigerstedt, "Die Physiologic des Kreislaufes, " 1893, p. 80, for 

 literature. 



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