244 VETERINARY PHYSIOLOGY 



is obviated by the lower cusp being mounted on the top of the 

 muscular septum upon which the pressure falls the other 

 cusps shutting down upon this one (fig. 110). 



The Flow of Blood through the Heart 



The circulation of blood through the heart depends upon 

 these differences of pressure in the different chambers and upon 

 the action of the valves. 



A fluid always flows from a point of high pressure to a point 

 of lower pressure. We may then consider the flow 



A. From Gt-reat Veins into Auricles. This occurs when the 

 pressure in the great veins is greater than the pressure in the 

 auricles (fig. 116). 



The pressure in the auricles is lowest at the moment of their 

 diastole. At this time there is therefore a great flow of blood 

 into them, but gradually this becomes less and less, until, when 

 the ventricles dilate, another fall in the auricular pressure 

 takes place and another rush of blood from the great veins 

 occurs. Gradually this diminishes, and by the time that the 

 auricles contract the flow from the great veins has stopped. 



The contraction of the mouths of the great veins, which 

 precedes the auricular systole, drives blood from the veins into 

 the auricles, and, as these enter into contraction, no flow from 

 the veins can occur and no back-flow from the auricles is 

 possible (fig. 116). 



B. From Auricles to Ventricles. As the ventricles dilate, a 

 very low pressure develops in them, and hence a great rush of 

 blood occurs from the auricles. During the passive stage of 

 ventricular diastole, the intra-ventricular pressure becomes 

 nearly the same as the auricular, and the flow diminishes or 

 may stop. When the auricles contract a higher pressure is 

 developed, and a fresh flow of blood occurs into the ventricles. 

 When the ventricles contract the auriculo-ventricular valves are 

 closed, and all flow of blood from the auricles is stopped (fig. 116). 



C. From Ventricles to Arteries. When the ventricles begin 

 to contract the intra-ventricular pressure is low, while the 

 pressure in the arteries is high and keeps the semilunar valves 

 shut. As ventricular systole goes on the intra-ventricular 

 pressure rises, until after about 0'03 of a second it becomes 



