480 CIRCULATION OF BLOOD AND LYMPH. 



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 con- 

 cerned. 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).* 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. 



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 

 will affect the electrodes at different times. This electrical varia- 

 tion of the contracting heart muscle may be shown easily by 

 means of the rheoscopic muscle-nerve preparation (see p. 99). If 

 the heart is exposed and the nerve of the preparation is laid over its 

 surface each ventricular systole is accompanied by a kick of the 

 muscle, since the nerve by connecting separated points acts as a 

 conducting wire for the current generated, and is stimulated, there- 

 fore, at each systole. Since the muscle-nerve preparation gives 

 only a simple contraction for each ventricular systole, we may 

 assume that this latter contraction is itself simple, that is, due 

 to a single stimulus. The electrical variation may be obtained also 

 by means of the capillary electrometer (p. 94), and since the move- 

 ment of the mercury in this instrument may be photographed the 

 results can be studied in detail. The variation is diphasic. If one 



* See Tigerstedt, " Die Physiologie des Kreislaufes," 1893, p. 80, for litera- 

 ture. 



