Upon the Motion oj the Mammalian Heart. 571 



massive. "Why are these two chambers developed in the heart ? The 

 function of the auricle is not to fill the ventricle ; the blood pumped from 

 auricle to ventricle at the end of the heart's diastole is but a fraction of its 

 full content. The function of the auricle is to save the veins from over- 

 distension. During a third of the ventricular cycle, during the period when 

 the ventricle is pouring blood into the body, the circulation is completely 

 obstructed at the auriculo-ventricular orifices. Yet the blood flow in the 

 veins continues ; the flow goes on into the expanding auricles. The 

 mechanical function of the auricle is that of a reservoir, to catch and hold 

 the content of the stream when a clam is thrown across it. It wants little 

 strength for this task ; its walls must stretch easily ; they have to pump 

 against a low resistance ; the muscle layer is therefore thin. 



The first sign of contraction is in the region of the opening of the superior 

 cava ; it quickly spreads to the mouth of the inferior cava. The mouths of 

 these veins tend to close before the main mass of auricular muscle is 

 involved, thus hindering or preventing regurgitation. The spread of the 

 contraction wave to the auricular muscle as a whole is expedited by the 

 central position of the node from which the wave starts, by the architecture 

 of the muscle bands which radiate from it in all directions, and, lastly, by 

 the relatively high rate of conduction which auricular muscle possesses 

 (1000 mm. per second). In the mammalian auricle the plan of distribution 

 is primitive. 



The wave travels through the auricle to the first part of that bridge which 

 joins the auricle and ventricle. Here there is delay and long delay, while 

 the auricle is emptying itself and the auriculo-ventricular valves float up 

 preparatory to closing. This delay occurs, so it is believed, in the auriculo- 

 ventricular node, a structure of very small muscle fibres (15). Leaving the 

 node, the wave passes to the bundle. The need of this structure and of its 

 branches and twigs has developed with the division of the ventricle into two 

 cavities. This special system of fibres is endowed with the highest order of 

 conducting power ; it is arranged that both ventricles and all their parts may 

 be thrown into contraction with the closest approach to simultaneity ; in that 

 lies a mechanical advantage which the ventricle as a driving muscle demands 

 for itself. The wave does not travel along the ventricular wall : were that so 

 and were its progress as rapid as in the wall of the auricle, its course, never- 

 theless, would be slow. It penetrates the wall ; its course in muscle is short, 

 and therefore a high rate of conduction in the muscle is not essential ; we 

 actually find it to be of a low order. 



The system of distribution in the thick-walled ventricle possesses a 

 further virtue in that it protects the laminated tissues from disruption. 



