268 TEXT-BOOK OF PHYSIOLOGY. 



the apex, turn upward and end in the same structures as do the fibers from 

 the anterior surface. The fibers from the base of the right ventricle termi- 

 nate in the structures of the left ventricle, while those from the left ventricle 

 terminate in the structures of the right ventricle. Longitudinal fibers are 

 also found on the inner surface. The transverse fibers are very abundant 

 and surround each ventricle separately though they are continuous with 

 each other across the septum. Between the superficial longitudinal and deep 

 transverse fibers there are several layers of fibers which possess varying 



FIG. 122. ARRANGEMENT OF VENTRICULAR MUSCLE-FIBERS. (After MacCallum.) I and 

 II, Superficial fibers of the left ventricle and conus arteriosus ; III, deep layers of the left ven- 

 tricle; LAV, mitral orifice; RAV, tricuspid orifice; PA, pulmonary artery. (From Hirschf elder.) 



degrees of obliquity. The general arrangement of the fibers is such as to 

 insure a complete and simultaneous discharge of blood from both auricles 

 as well as from both ventricles (Fig. 122). 



THE MUSCLE CONNECTION BETWEEN THE AURICLES AND 



VENTRICLES. 



The Muscle Band of His, or the Auriculo-ventricular Bundle. In 



the mammalian heart there is no continuity of the muscle-fibers across the 

 auriculo-ventricular groove, uniting auricles and ventricles, such as exists in 

 the frog or turtle heart. The muscle-fibers of the auricles and ventricles are 

 completely separated from each other by the transverse fibrous septum to 

 which they are attached. This fact has for a long time made it difficult to 

 understand how the contraction process which begins in the auricles (and to 

 which there will be occasion to refer in subsequent paragraphs) is conducted 

 to the ventricles. The physiologic necessity for the existence of a muscle 

 connection between the auricles and ventricles led to a series of investigations 

 which have resulted in the discovery of an elaborate system of muscle-fibers 

 by which they are united both anatomically and physiologically. 



In 1893 Wilhelm His, Jr., discovered the existence of a band or bundle 

 of muscle-fibers which apparently took its origin from the posterior part of 

 the right side of the auricular septum, from which point it passed forward 

 just above the auriculo-ventricular septum to a point near the aortic opening, 

 where it divided into two portions, a right and a left, of which the latter 

 apparently ended in the basis of the aortic leaflet of the mitral valve. This 

 bundle has been termed "the muscle-bundle of His." In 1904 Retzer and 

 Braunig, working independently, corroborated the existence of this bundle 

 and described its anatomic course more completely. The investigations of 

 Braunig led to the conclusion that this bundle of muscle-fibers which was 



