EXCITATION OF THE HEART 



289 



guides to it have been included in the references to 

 tliis chapter (27, 48, 64, 81, 83, 84, 128, 129). Some 

 consideration will be gi\en later to the physiological 

 evidence on this point. 



Atrioventricular Condiution System 



The A-\' node (127) is a mass of specialized tissue 

 anatomically resembling the sinus node. The A-V 

 node lies aboxe the inter\cntricular septum in the 

 interatrial septum, approximately above the central 

 portion of the tricuspid valve. There is some question 

 about the anatomy of the cells on the border between 

 ordinary atrial musculature and the cells of the A-V 

 node. It appears that on this boundary there are 

 cells which are intermediate between the atrial and 

 the nodal cells and 'or cells which are somewhat 

 different from either and smaller. Indeed, several 

 "nodes'" have been named in this area (8, 27, 68). 

 Also, there is some feeling that a specific internodal 

 (S-A to A-V) pathway exists. The node, as mentioned, 



FIG. 3. Distribution of the left bundle in goat heart as shown 

 through injection. Left bundle emerges beneath the aortic 

 valve as a single bundle. It gives rise to two branches which 

 cross the cavity, one running to the anterior and one to the 

 posterior papillary muscle, and to two branches which run to 

 the endocardium between the papillary muscles. Note the 

 numerous fibers crossing the ca\'ity and those forming an ex- 

 tensive endocardial network. [From Aagaard & Hall (i).] 



lies somewhat posteriorly and at the right border of 

 the interatrial septum, but at times the node is appar- 

 ently displaced from this rightward position (27). In 

 man the node is about 2 mm in width and height and 

 3 mm long, and gives rise to the common bundle. 



-•1-1' Conduction Svstei, 



Lower Forms 



The atrioventricular ring of the turtle contains 

 cells resembling the Purkinje fibers of other hearts 

 (96). These cells cross the A-V ring and apparently 

 conduct the impulse from the atrium to the ventricle. 

 They are similar to the atrioventricular plug in the 

 frog (16, 88) and fowl (86), and the atrioventricular 

 bundle in the reptile (87). Concerning the last there 

 is some disagreement (37). 



Purkinje Fibers of the J'entric/e 



The common bundle and the right and left bundles 

 in most animals consist of large Purkinje cells that 

 can be easily identified histologically (17, 64, 97, 135). 

 In the mammal the common bundle commences at 

 the A-V node, runs to the left, and passes through 

 the membranous septum. After passing through this 

 septum, a little in front of the septal leaflet of the 

 tricuspid valve, the common bundle breaks up into 

 the left and right bundles. The riglit bundle moves 

 onto the endocardial surface of the septum and 

 proceeds, with a slight curvature, to the anterior 

 papillary muscle of the right \entricle (fig. 2). Here 

 it arborizes. The arborizations apparently run to 

 both the septal surface and the free wall of the right 

 ventricle. The left bundle moves through the mem- 

 branous septum and spreads, fanwise, beneath the 

 aortic x'alves on the left, so that no clear bundle is 

 discernible for i or 2 mm below the aortic vah'e. The 

 bundle then passes down the septum and gives rise to 

 several branches which conduct the impulse into the 

 ventricles (fig. 2). Some branches cross the basal left 

 cavity as they pass from the septal surface to the 

 papillary muscles on the left. In man and in the dog 

 there appear to be two main branches of the left 

 bimdle, which run to the anterior and posterior 

 papillary muscles, respectively. In other animals (i, 

 71, 136, 137) it appears that the left bundle is not so 

 discretely separated into two branches. There may 

 well be other branches of the left bundle in man and 

 in dog. The subsidiary branches may lie between the 

 two major branches as they descend the left septal 

 surface (fig. 3). The two major branches of the left 



