THE CIRCULATION OF THE BLOOD. 269 



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 was 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-ventric- 

 ular 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 

 limbs is enclosed by a layer of connective tissue which isolates it from the 

 musculature of the ventricular septum as far as the lower third of the ven- 

 tricular cavities. In this region they divide into a number of bundles, some 

 of which enter the papillary muscles, while others, forming tendon-like 

 strands, branch freely beneath the endocardium and spread in all direc- 

 tions over the entire inner surface of the ventricle and enter into histologic 

 connection with the true cardiac muscle-fibers. 



The fibers composing this system, and termed by Tawara from its sup- 

 posed function the " conduction system " are historically different from the 

 cardiac fibers, in so far as they are poorer in sarcoplasm and similar in their 

 appearance to embryonic muscle-fibers. In the auricular portion of the 

 bundle the fibers exhibit a more or less reticular arrangement; in the ven- 

 tricular portion, the fibers are more regularly arranged, are richer in sarco- 

 plasm and present a number of fibrillae near their periphery. In associa- 

 tion with the muscle fibers composing the auriculo-ventricular bundle there 

 is a special collection of nerve cells and nerve fibers. Their function is 

 unknown. 



The ultimate termination of the system, beneath the endocardium, con- 

 stitutes the so-called Purkinje fiber layer. In the sheep, calf, and in other 



