194 



THE CIRCULATION OP THE BLOOD 



significance in connection with their function of tightening the chordae 

 tendinese so as to prevent any bulging of the flaps of the auriculoven- 

 tricular valve into the auricles when, at the beginning of the presphygmic 

 period, the high intraventricular pressure is brought to bear on their 

 under surfaces. After starting at this point in the ventricle, the con- 

 traction wave seems to spread farther through the ventricular muscle at 

 a fairly uniform rate. 



Investigation of this problem by means of the galvanometer has been 

 technically a very difficult matter. According to the researches of 

 Lewis and his associates, however, it appears that, when a series of 

 nonpolarizable electrodes are placed at various parts of the outer 



Fig. 55. Diagram of experiment by Lewis showing the times at which the excitation wave 

 appeared on the front of the heart relative to the upstroke of R in lead II. R.A., right appen- 

 dix; D.B.L., descending branch of left coronary artery. (From Thomas Lewis.) 



aspect of the ventricle, and comparison made of the moments at which 

 the cardiac impulse arrives, as judged by the appearance of the excita- 

 tion wave relative to R. in a standard electrocardiogram, it has been 

 found that the time of arrival bears no relationship to the anatomical ar- 

 rangement of the muscle bundles of the ventricle. It arrives early and 

 simultaneously over an area of the surface near the anterior attachment 

 of the wall of the right ventricle to the septum. It arrives late at 

 the base of the right ventricle and in the part near the posterior 

 intraventricular groove. The actual rate of spread is moreover much 

 greater than the transmission rate through ventricular muscle. His- 

 tological examination has shown that the branches of the right division 



