THE HEART BEAT. 541 



tion travels over the heart with a certain velocity, which for the 

 human heart has been estimated at 5 m. per second (Waller), and 

 for the rabbit's heart (Gotch) at 3 m. per second. In regard to 

 the course of the wave there have been great differences of opinion 

 among different investigators. While the older view assumed as a 

 matter of course that the contraction begins at the base and 

 passes toward the apex, the newer knowledge in regard to the con- 

 duction system between auricles and ventricles seems to indicate 

 from the anatomical side that the auriculoventricular bundle 

 spreads out upon the papillary muscles in the interior of the ven- 

 tricle, and that, therefore, the contraction of the ventricles may 

 begin with the papillary musculature and thence spread to the 

 oblique and circular fibers of the ventricles.* This anatomical 

 indication has been confirmed experimentally by some investiga- 

 tors and contradicted by others. Observations upon the papillary 

 muscles seem to show that they are not the last portion of the mus- 

 culature to enter into contraction, but whether the ventricular 

 systole begins in them is not so clear, although the trend of recent 

 opinions seems to be toward this view. The course of the contrac- 

 tion wave has been studied chiefly by means of the accompanying 

 electrical variation, and the conflicting results obtained from this 

 method are referred to briefly in the succeeding paragraph. Be- 

 tween the auricular and ventricular contractions there is a per- 

 ceptible interval, which, for the human heart, can be estimated 

 from a study of the records of the jugular pulse. The interval 

 between the a and the c waves, the a-c interval, as it is called, 

 marks the time intervening between the contraction of the auricles 

 and of the ventricles. This interval may be valued at 0.2 sec. or 

 less (0.12 to 0.2 sec.), and is due chiefly to the time necessary for 

 the excitation wave to pass over the conducting system between 

 auricles and ventricles. In all probability the conduction in this 

 system is slower than in the musculature of the auricles or ven- 

 tricles. In the dog, for example, the interval between auricular 

 and ventricular contraction is about 0.1 sec. Since the connecting 

 auriculoventricular bundle has a length of 10 to 15 mm., the 

 velocity of the conduction through this bundle must be about 

 10 to 15 cm. per second. 



The Electrical Variation. The contraction of the heart 

 muscle, like that of skeletal muscle, is accompanied or preceded 

 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 or, to be more accurate, the ad- 



* Bering, "Pfliiger's Archiv f. d. ges. Physiol.," 126, 225, 1909. 



