THE CIRCULATION OF THE BLOOD 271 



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 

 septum, the ultimate distribution and termination of these limbs were not 

 clearly determined. Retzer estimated that this bundle was 18 mm. long, 

 2.5 mm. broad, and 1.5 mm. thick. By these investigators this bundle was 

 termed the "auriculo-ventricular bundle." 



In 1906 Tawara published the results of an extended series of investiga- 

 tions made on the embryonic and adult hearts of many mammals including 

 man, which resulted in a further increase of knowledge concerning the 

 development, anatomic course, and histologic features of this bundle, and 

 established beyond doubt that it is the pathway along which the contraction 

 process is conducted from the auricles to the ventricles. 



A brief summary of Tawara's account of this bundle is as follows: It 

 arises near the opening of the coronary sinus where it is connected with the 

 true auricular fibers. From their origin the fibers converge to form a dis- 

 tinct bundle which then passes forward on the right side of the auricular 

 septum between the lower edge of the fossa ovalis and the auriculo-ventricu- 

 lar septum; just above the insertion of the median cusp of the tricuspid 

 valve the bundle presents a very complicated network of muscle-fibers which 

 has been designated as a knot or the auriculo-ventricular node or the node 

 of Tawara; from the anterior portion of the node a bundle of fibers turns 

 downward and penetrates the auriculo-ventricular septum, beyond which it 

 passes below the pars membranacea septi to the upper limit of the muscle 

 portion of the ventricular septum. It then divides into two limbs or 

 branches which descend on either side of the septum under the endocar- 

 dium, the right limb lying somewhat deeper than the left. Each of these 



