PROPERTIES OF HEART MUSCLE 



179 



it through the lesser circulation to the left auricle. Should the two ventricles 

 not expel exactly the same quantity of blood at a systole or in a unit time 

 we except accidental disturbances the blood would collect somewhere in the 

 vascular system. Under the assumption that the mean pressure in the pul- 

 monary arteries of man is equal to that of the dog, we obtain for the work of 

 the right ventricle 14.24 to 28.48 g-m. 



In connection with the subject of the blood movements in the arteries we 

 shall discuss more fully how the work of the heart depends upon changes in the 

 vessels and upon their degree of fullness. 



9. PROPERTIES OF HEART MUSCLE 



A. THE NATURE OF THE CARDIAC CONTRACTION 



If the contraction curve of an empty heart be recorded graphically, one 

 observes an unmistakable resemblance to an ordinary muscular twitch produced 

 by a single stimulus. We have in the 

 action current of the heart (cf. page 48) 

 a means of testing this inference. This 

 test can be applied to the human sub- 

 ject also, if symmetrical points of the 

 body's surface be connected with a gal- 

 vanometer, for the electrical currents 

 generated by the heart's activity diffuse 

 according to the usual laws throughout 

 the entire body (cf. Fig. 63). Fig. 64 

 represents the action current of the 

 human heart as recorded by the excur- 

 sions of the capillary electrometer. An 

 upward stroke signifies that the base of 

 the heart is negative to the apex. The 

 ventricular systole begins with the point 

 R; there follows a negative stroke S (the 

 apex negative to the base), and finally, 

 after an interval, a positive stroke T 

 (base negative to apex). From this 

 curve of electrical variations we may 

 infer that the contraction begins at the 

 base and proceeds from there to the apex; 



for a certain time (a portion of the stretch from S to T) the ventricle is contracted 

 in all its parts, so that the base and the apex exhibit 110 difference of potential. 

 The contraction ceases sooner at the apex than at the base which in all proba- 

 bility is due to the return course of the muscular fibers (page 163) and the 

 base becomes once more negative to the apex. The interval of time between 

 the beginning R and the end T is about 0.32 second, which corresponds fairly 

 well to the duration of a ventricular systole. 



From all this it appears that the ventricular systole is comparable with 

 a simple muscular contraction and cannot be regarded as a summated con- 

 traction (cf. Chapter XV). 



FIG. 63. Schematic representation of varia- 

 tions of electrical potential associated 

 with the beat of the human heart, and 

 their distribution in the body, after 

 Waller. 



