THE PHYSIOLOGY OF THE HEARTBEAT 



183 



heart arises as the so-called cardiac tube. As development proceeds, 

 diverticula grow out from the walls of this tube to form the auricles and 

 ventricles. In the comparatively simple heart of the turtle these dispo- 

 sitions of the auricles and ventricles in relationship to the cardiac tube 

 are more or less evident even in the fully developed heart, particularly 

 in the case of the auricles (Fig. 48) ; but in the heart of the higher 

 mammalia it is impossible by superficial examination alone to show any 

 remains of the primitive cardiac tube. More careful anatomic investiga- 

 tions during recent years have, however, shown that it exists in the form 

 of certain definite structures composed of tissue histologically quite dif- 

 ferent from that of the rest of the heart, and disposed in such a manner 



TH 



TH 



Fig. 48. Heart of tortoise as suspended. B, body of tortoise; TH. threads to levers; CL, clamp 

 holding aorta; A, auricle; C, coronary nerve; S, sinus; V, ventricle. (From Gaskell.) 



as would indicate not only that it is derived from the primitive cardiac 

 tube, but also that it is the main pathway along which the beat is 

 transmitted. 



This primitive cardiac tissue is much better developed in certain re- 

 gions than in others, the first portion of it to be discovered being that 

 known as the auriculoventricular node, or the node of Stanley Kent* (Figs. 

 49 and 50) . This structure is found at the base of the interauricular sep- 

 tum on the right side and near its posterior margin. It exists as a collection 

 of peculiar small primitive cells and fibers, and is continued forward and 

 downward as a bundle of the same peculiar tissue to the interventricular 

 septum, where, a little in front of the attachment of the septal valve, it 



*The discovery of this node is often erroneously attributed to His, and called after his name. 



