THE CIRCULATION OF THE BLOOD AND LYMPH 135 



of securing a certain interval between the systoles of successive 

 divisions. In any case, since we know that the velocity of the 

 nerve-impulse is very different in different varieties of nerves, the 

 question cannot be decided by general arguments of this kind. 

 In Limulus, as a matter of fact, the velocity in the intrinsic 

 heart nerves is only one-tenth as great as in the ordinary motor 

 (limb) nerves of the animal (Carlson) . 



In the mammalian heart the alleged absence of muscular con- 

 nection between the auricles and ventricles was long the founda- 

 tion of the general belief that the link was a nervous one. Cer- 

 tainly there is no dearth of nerves which might serve as such a 



FIG. 54. RIGHT AURICLE AND VENTRICLE OF CALF, TO SHOW AURICULO - 

 VENTRICULAR BAND (KEITH). 



i, Central cartilage ; 2, main auriculo -ventricular bundle ; 3, auriculo- ventricular 

 (A-V) node ; 4, right septal division of the bundle ; 5, moderator band ; 6, medial or 

 septal cusp of tricuspid valve ; 8, coronary sinus. 



bridge. But it has been shown (Kent, His, etc.) that in the mam- 

 malian heart, too, a slender band of muscular fibres, arising at a 

 definite point near the coronary sinus on the right side of the 

 interauricular septum below the fossa ovalis, passes forwards 

 and downwards through the fibrous ring between the auricles and 

 ventricles under the septal cusp of the tricuspid valve. It then 

 divides into two branches, one for each ventricle, which run 

 down the interventricular septum towards the apex, spreading out 

 as the Purkinje fibres or their equivalent, to blend at last, with 

 the ordinary muscle of the ventricles, and particularly of the 



