THE CIRCULATION. 



257 



The time of the normal block to the transmission of the wave 

 of impulses from the auricle to the ventricle is about one-fifth of a 

 second. With very little compression of the auriculo-ventricular 

 bundle in animals by the clamp of Erlanger there is simply an 



Fig. 76a- Right Auricle and Ventricle of the Calf. (KEITH.) 



1, Central cartilage. 2, Bundle of His or the auriculo-ventricular bundle 

 dividing into right and left septal branches (av bundle). 3, av, Node. 4, Right 

 division of the bundle of His. 5, The right branch of the bundle of His breaks 

 up into smaller strands which pass to the papillary muscles (4 and 6) seated 

 on the septum. Arising from the group of papillary muscles are the terminal 

 branches of the bundle of His, which, especially on the right side, takes the 



form of the small "so-called moderator" bands and pass out to all parts of the 

 ventricular wall and fuse with the ventricular muscles. 8, Orifice of coronary 

 sinus, which is a good guide to the position of the av node, which lies below 

 and to the right. Microscopically the av node has peculiar branched cells the 

 main bundle contains large pale cells with large nuclei and the peculiar Purkinje 

 cells found in the septal divisions and their terminal ramifications, especially 

 in the "so-called moderator" bands. The sino-auricular node portion of sinus 

 venosus is found where the superior vena cava joins the taenia terminalis of 

 the right auricle. This sino-auricular node is identical in structure with the 



av node. These nodes are in muscular connection with each other, probably 

 have the same function, and are remains of the primitive cardiac tissue. 



increase of the normal pause during which the wave from the auricle 

 reaches the ventricle. This delay may be caused either by a slow trans- 

 mission of the auricular impulse through the bundle of His which 

 has been compressed, or to a decrease in the intensity of the impulses 



