404 VETERINARY PHYSIOLOGY 



Nevertheless, the occlusion of this orifice is apt to be in- 

 complete when the right side of the heart becomes in the 

 least over-distended, and this gives rise to what may be 

 called a safety-valve action from the right ventricle, which 

 prevents over-distension. 



The auriculo-ventricular valves are open duHng the 

 whole of the cardiac cycle, except during the ventricular 

 systole (fig. 173). 



B. Semilunar Valves. — Before the ventricles contract 

 these valves are closed and the various segments pressed 

 together by the high pressure of blood in the arteries. 



As the ventricles contract the pressure rises, until the 

 intra- ventricular pressure becomes greater than the pressure 

 in the arteries. This is the "presphygniic period. Then the 

 cusps of the valves are thrown back and remain open until 

 the blood is expelled. When the outflow of blood is com- 

 pleted, the cusps are again approximated by the pressure of 

 blood in the arteries. As relaxation of the ventricles occurs, 

 the intra-ventricular pressure becomes suddenly very low, 

 and the high pressure of the blood in the arteries at once 

 falls upon the upper surfaces of the cusps, which are thus 

 forced together and downwards, and completely prevent any 

 back-flow of blood. 



The prejudicial effect of too great pressure upon these 

 cusps is obviated by the lower cusp of each being mounted 

 on the top of the muscular septum upon which the pressure 

 falls — the other cusps shutting down upon this one (fig. 



167). 



The semilunar valves are open only during the floiu of 

 blood from the ventricles to the arteries in the second and 

 third periods of ventricular systole (fig. 173). 



8. The Flow of Blood through the Heart. — The circula- 

 tion of blood through the heart depends upon the difterences of 

 pressure in the diiferent chambers and upon the action of 

 the valves. 



A. From Great Veins into Auricles. — This occurs when 

 the pressure in the great veins is greater than the pressure 

 in the auricles (fig. 173). 



