THE HEART BEAT. 479 



course of the fibers in the ventricles has been difficult to make 

 out and several more or less different accounts have been pub- 

 lished. It is clear from even a casual examination that the 

 superficial fibers are common to both ventricles. They may be 

 considered as arising from the auriculo- ventricular ring in one 

 ventricle to pass in a spiral course to end in the papillary 7 muscles 

 and through their tendons in the auriculo- ventricular ring of the 

 other ventricle. Those that begin on the outer surface in one ven- 

 tricle end on the inner surface in the other. This arrangement is rep- 

 resented in Fig. 202, / and II. The contractions of these bands of 

 fibers would tend not only to diminish the cavities of the ventricles 

 from side to side, but also to bring the apex and base together and to 

 rotate the apex from left to right. Beneath these superficial fibers 

 lie thicker bands, the fibers of which have a more transverse course. 

 According to MacCallum,* these fibers form three flat bands which 

 pass in the form of a scroll from one ventricle through the septum 

 into the other, as shown in Fig. 202, ///. The band that lies most 

 superficially in the left ventricle at its origin lies deepest in the right 

 ventricle. The effect of the contractions of these bands should be 

 to compress the cavities of the ventricles in the lateral diameters. 

 In addition to these two main systems of fibers there are other less 

 prominent bands belonging entirely to one ventricle. A matter 

 of very great physiological interest in connection with the invariable 

 sequence of the heart beat has been the question of the existence of 

 a direct muscular connection between the auricles and ventricles. 

 While such a connection exists obviously in the lower animals, 

 frogs, terrapins, in the mammalia there is a conspicuous tendinous 

 ring at the auriculo- ventricular groove which has been believed by 

 many to make a complete separation between auricles and ventricles. 

 Several observers, however, have shown recently that there is a 

 muscular connection in the heart of man and of a number of mam- 

 malia.f The chief connection is described as a bundle of fibers, 

 auriculo- ventricular bundle, which springs from the right side of the 

 interauricular septum, runs obliquely through the connective tissue, 

 and ends in the muscle of the ventricular septum under the origin 

 of the aorta. 



The Contraction Wave in the Heart. The muscular contraction 

 of the heart beat begins at the mouths of the great veins opening into 

 the auricles, and thence passes to the auricles first and subsequently 

 to the ventricles. The continuity of the muscular tissue enables us 

 to understand how this contraction passes quickly from cell to cell 

 in the direction of the muscular fibers. In the mammalian heart 



* MacCallum, "Contributions to the Science of Medicine," dedicated to 

 W. H. Welch, p. 307, Baltimore, 1900; contains also the literature. 



t See Retzer, "Archiv f. Anatomic," 1904, p; 1; and Braeunig, "Archiv 

 f. Physiologic," 1904, suppl. volume, p. 1. 



