THE HEART 39 



modified owing to the extension of the bases of the ventricles 

 backwards under the basal wall of the auricular canal. By this 

 means the mesial fold has come to rest at the base of the inter- 

 auricular septum on the right side just about the top of the inter- 

 ventricular septum. Most of the fibres representing the auricular 

 part have become indistinguishable from the other auricular 

 tissue, but the circular fibres of the auriculo-ventricular groove 

 may be held to represent them. However at the spot referred 

 to above, namely, at the base of the interauricular septum on the 

 right side, a portion of the ring has remained undifferentiated. This 

 is the node of tissue termed the " auriculo-ventricular or A-V node," 

 and which is similar in structure to the " sino-auricular node." 



The in vagina ted portion of the auricular canal is of interest, 

 since in the lower type of heart (Fig. 1, 4.4) it forms a muscular 

 connection between the auricular and ventricular portions of 

 the heart. Has this invaginated portion any homologue in the 

 mammalian heart, since if it have then a muscular connection 

 must exist between the auricles and ventricles of the mammalian 

 heart ? For a long time this question was answered in the negative. 

 The anatomists taught that in the mammalian heart the auricles 

 were absolutely separated from the ventricles by fibrous tissue, 

 so that no such muscular connection could exist. In 1893, how- 

 ever, His, jun., described a muscular connection between the 

 auricles and ventricles. Stanley Kent also in the same year 

 came to the conclusion that the auricle and ventricle were con- 

 nected by muscle. Recently in 1904 Retzer and also Braunig 

 corroborated the observation of His in certain mammalian hearts, 

 but not in all. Tawara in 1905 published a most elaborate and 

 accurate account of this muscular connection, its extensive nature 

 and its connection with the Purkinje fibres. Both he and other 

 observers have found it in all the mammalian hearts examined, 

 so that now there appears to be no doubt that such a muscular 

 connection exists in all mammalian hearts. Keith and the writer 

 have shown that, as might be expected, it is the homologue of the 

 invaginated portion of the auricular ring. As later we shall have 

 occasion to refer to this muscular connection, it will perhaps be 

 well to describe it in some detail. 



The Muscular Connection between Auricle and Ven- 

 tricle in the Mammalian Heart. The Auriculo-Ventricular 

 (A-V) Bundle. This connection is sometimes called after His, 



