396 



VETERINARY PHYSIOLOGY 



of primitive tissue along the back of the auricular septum, 

 and then to the mouths of the great veins partially occluding 

 them, then over the rest of the auricles, which become 

 smaller in all directions and seem to be pulled down towards 

 the ventricles. The contraction of the auricles in mammals 

 is not accompanied by so marked a dilatation of the 

 ventricles as in the frog. 



(2) The wave of contraction in the auricles is propagated 

 to the ventricles through the auriculo-ventricular band, and 

 when this is diseased the passage of the wave of contraction 

 is interfered with. 



(3) As the ventricles contract, the auricles relax. 



The ventricular contraction develops suddenly, lasts for 

 some time, and then suddenly passes oft. The wave of 

 contraction is chiefly conducted by the primitive tissue which 

 runs on the interior aspect of the ventricles. Lewis has shown 



Avmcu5 



V€hTRICLE:> 



Fig. 170. — Scheme of the Cardiac Cycle in the Human Heart. A.8., auri- 

 cular sj'stole ; V.S., ventricular systole ; P., pause. 



that, if this layer is divided, conduction is markedly delayed, 

 while, if the outer fibres of the ventricles are cut, no marked 

 delay occurs. 



(4) The contraction of the ventricles is followed by 

 a period during which both auricles and ventricles remain 

 relaxed. This is called the pause of the cardiac cycle. 



The cardiac cycle in mammals may be represented as in 

 fig. 170. 



3. Duration of the Phases. — Ventricular systole lasis 

 three times as long as auricular systole. 



The duration of these two phases in relationship to the 

 pause varies very greatly. Whatever may be the rate of 



