THE CIRCULATION OF THE BLOOD 273 



interval of at least 0.013 sec. elapses between the beginning of the right 

 and the beginning of the left auricular systole. Though the excitatory 

 process probably passes by direct continuity of the fibers common to 

 both auricles, there yet appears to be a special conduction bundle or 

 band of fibers stretching from the right auricle to the base of the left 

 auricle (Fig. 115). In the dog's heart this interauricular band, "origi- 

 nates about five millimeters to the mesial side of the auriculo-caval angle 

 and in a direct line with it. This extremity of the band is immediately 

 back of the aorta and spreads in a fan-like manner on to the mesial (right) 

 auricular wall and the root of the superior vena cava. From this origin 

 the band sweeps in an almost straight line to the base of the left auricular 

 appendage." The length of the band is about 20 millimeters. 



In this situation it appeared that the band might serve as a more 

 direct path for the excitation process. If this were the case its compres- 

 sion ought to be followed by a lengthening of the interauricular interval. 



FIG. 115. VENTRAL VIEW OF THE AURICLES IN THE DIASTOLIC STATE. The interrupted 

 lines indicate the location and course of the interauricular band. (Bachmann.) 



With a view of demonstrating the truth of this supposition Bachmann 

 made a series of experiments devised to crush this bundle while recording 

 the auricular contractions. Successful crushing of the bundle, as shown by 

 postmortem examinations, lengthened the interauricular time from its 

 average of 0.013 sec. to 0.05 sec. on the average. A comparison of the 

 results of these experiments show that the conduction time between the 

 auricles was lengthened from 3 to 4.6 times. The presence and function 

 of this band seems therefore to be established. Though perhaps not the 

 only, it is the primary or more direct path for the conduction of the 

 excitation process from the right to the left auricle. 



THE MECHANICS OF THE HEART 



Methods of Observation. The movements of the heart, as well as 

 many phenomena connected with the flow of blood through its cavities, have 

 been determined by observation of, and experimentation on, the exposed heart 

 of a mammal e.g., dog, cat, rabbit supplemented and corrected by experi- 

 ments on the heart in its normal relations. Valuable information as to the 

 18 



