THE HEART BEAT. 541 



the heart. There is, however, a pause or interruption in the pas- 

 sage of this wave at the sino-auricular junction, at the auriculo- 

 ventricular junction, and at the bulbo ventricular junction, so that 

 the contraction of each chamber is marked off as a separate oc- 

 currence. In the human heart and the mammalian heart in 

 general we are accustomed to distinguish only the auricles and 

 ventricles, but physiological and anatomical studies combined 

 have shown that in such hearts a remnant of the sinus venosus is 



Fig. 224. A generalized type of vertebrate heart, combining features found in the eel, 

 dogfish and frog (Keith): a, Sinus venosus and veins; b, auricular canal; c, auricle; d, ventricle; 

 e, bulbus cordis;/, aorta; 1-1, sino-auricular junction and venous valves; 2-2, canalo-auricular 

 junction; 3-3, annular part of auricle; 4-4, invaginated part of auricle; 5, bulboventricular 

 junction. 



found in the right auricle, particularly in the area lying between 

 the openings of the venae cavse and round the coronary sinus. A 

 special collection of this tissue which lies " in the sulcus terminalis 

 just below the fork formed between the junction of the upper sur- 

 face of the auricular appendix with the superior vena cava" has 

 been described by Keith and Flack, and is designated as the sino- 

 auricular node. The beat of the heart begins in this tissue, as in 

 the case of the hearts of the lower vertebrates, and spreads directly 

 to the auricular muscle. The matter of greatest interest in con-< 

 nection with the different chambers has been the nature of the auric- 

 ulo ventricular junction. In the mammalian heart tendinous tissue 

 develops in this region, and for a long time it was supposed that 

 there was no muscular connection between auricles and ventricles. 

 In recent years, however, it has been shown most satisfactorily that 

 there is a peculiar band of cardiac muscle or modified muscle, known 

 usually as the auriculoventricular bundle, which connects auricle and 



