256 PHYSIOLOGY. 



enter the anterior and posterior papillary muscles. When they get to 

 the papillary muscles, some of the fibers pass beyond them, enter the 

 parietal wall of the ventricle, where they send branches upwards and 

 downwards under the endocardium and lining of the interior surface 

 of the ventricles to fuse with the cardiac muscle. The auriculo- 

 ventricular bundle throughout its whole extent is peculiar as regards 

 structure when compared with ordinary cardiac muscle-fibers by the 

 fact of a small development of sarcoplasm. The embryonic fibers 

 described by Purkinje, of Breslau, in 1845, in the sub-endocardial 

 layers of the ventricle are really the main branches of the ventricular 

 part of the auriculo-ventricular bundle. The Purkinje fibers are pale 

 in stained preparations on account of the diminished amount of 

 fibrillary substance and by their well-marked connective-tissue sheath 

 are easily distinguished from other ventricular fibers. Moenckeberg 

 in the main facts corroborates the findings of Tawara. 



On the inner surface of the ventricles have been found tendinous 

 structures. Tawara thinks they are nothing else than branches of 

 the auriculo-ventricular bundle, which are normal in many lower 

 animals but abnormal in man. Moenckeberg has divided the so-called 

 "tendons" into four groups, as regards the left ventricle: (1) fibers 

 which contain no muscular fiber, actually abnormal tendons; (2) fibers 

 which are ventricular muscle fibers; (3) fibers which are exclusively 

 of the auriculo-ventricular bundle; (4) fibers which are ventricular 

 and mixed with auriculo-ventricular bundle. The first two kinds do 

 not have anything to do with the bundle of His; the last two are 

 fibers of the left branch of the auriculo-ventricular bundle which are 

 abnormal. 



Keith and Flack have shown the existence at the junction of 

 the superior vena cava and the right auricle of a ring of peculiar 

 muscle-tissue which they named the sino-auricular node and which has 

 a structure similar to that of the auriculo-ventricular bundle and node, 

 and is furnished with a separate blood supply from both coronary 

 arteries. It also contains nerve-cells and their fibers. The connection 

 between this ring of tissue and the auriculo-ventricular node is still 

 uncertain, but appears to be by means of the auricular muscle-fibers. 



Conduction of the auricular impulse is much slower in the bundle 

 of His than in the other muscle-fibers of the heart. Clamping this 

 bundle in animals varies its conductivity. The experiments of Hering 

 and Erlanger have shown that along this "auriculo-ventricular bundle 

 of His" the wave of stimulation is carried from the auricle to the 

 ventricle. 



