THE CIRCULATION OF THE BLOOD 271 



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 

 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. 117). 



FIG. 117. 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, pulmonic artery. (From Hirschf elder.) 



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 

 constantly present in all animals examined, including man, began on the 

 right side of the auricular wall below the fossa ovalis, from which point it 

 passed forward, and anteriorly penetrated the auriculo-ventricular septum 

 to become connected with the musculature of the ventricular septum just 

 below the pars membranacea septi. Though both these observers state that 

 the bundle divides into a right and left limb as it enters the ventricular 



