272 TEXT-BOOK OF PHYSIOLOGY 



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 histologically 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 sar- 

 coplasm and present a number of fibrillse near their periphery. In asso- 

 ciation 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 

 animals these fibers are abundant and readily recognized; though they are 

 not so well developed, they are nevertheless present and extensively dis- 

 tributed in the human heart. 



The Keith-Flack Node or the Sino-Auricular Node. This is a 

 small body, discovered by the investigators whose names it bears, situated 

 in the sulcus terminalis "just below the fork formed by the junction of the 

 upper surface of the auricular appendix with the superior vena cava." It 

 appears to be a remnant of primitive muscle tissue at what was formerly the 

 junction of the sinus venosus and the auricle. In its structure it resembles 

 the auriculo-ventricular (Tawara's) node, in that it consists of peculiar 

 muscle-fibers, nerve-cells, and nerve-fibers enclosed by connective tissue. It 

 is also provided with an abundant blood-supply. In the human heart, the 

 muscle-fibers of this remnant are striated, possess well marked and elongated 

 nuclei and are plexiform in arrangement. From the node the muscle-fibers 

 extend downward along the sulcus terminalis for about two centimeters. 

 The thickness of the bundle is about two millimeters. Superiorly the node 

 appears to be connected with or continuous with fibers in the superior vena 

 cava; inferiorly it is connected with the true auricular fibers. The dissection 

 of this node shows that the terminal branches of the vagi and sympathetic 

 nerves are in histologic relation with the nerve-cells. The situation, struc- 

 ture and relations of this neuro-muscle node appear to justify the assump- 

 tion that it is directly concerned in the initiation of the heart-beat. 



The Inter-auricular Band. It is generally stated that the left 

 auricle contracts simultaneously with the right auricle notwithstanding its 

 greater distance from the sino-auricular node. Though a difference 

 in time between the beginning of the contraction of the two auricles has 

 been noted by different observers the time relations and the paths of 

 conduction have not been demonstrated until recently. In 1916, Bach- 

 mann (American Journal of Physiology, vol. xli, p. 309) by taking sim- 

 ultaneous tracings of both auricular contractions in the living animal 

 (dog) established that the time of the onset is not synchronous but that an 



