UW 



CIECULATION 243 



may be somewhat pulled backwards and to the left. The 

 posterior cusp is pulled forwards against the anterior. 



On the right side the infundibular cusp of the tricuspid 

 valve is stretched between the superior and inferior papillary 

 muscles, and is thus pulled towards the bulging septum, 

 against which it is pressed by the increasing pressure inside 

 the ventricles. The posterior cusp has its anterior margin 

 pulled forward and its posterior margin backwards, and is 

 thus also pulled toward the septum. The septal cusp 

 remains against the septum. The greater the pressure in 

 the ventricle the more firmly are these cusps pressed against 

 one another and against the septum, and the more completely 

 is the orifice between the auricle and the ventricle closed. 

 On the right side of the heart other factors play an import- 

 ant part in occluding the orifice ; the muscular fibres which 

 surround the auriculo-ventricular opening contract, while the 

 papillary muscles pull the auriculo-ventricular ring downwards 

 and inwards through the chordte which are inserted into it. 



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, giving rise to a safety-valve action from 

 the right ventricle. 



The auriculo-ventricular valves are open during the whole 

 of the cardiac cycle, except during the ventricular systole 

 (fig. 116). 



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 aorta. 



As the ventricles contract the pressure in them rises, until 

 the intra- ventricular pressure becomes greater than the pressure 

 in the arteries. Instantly the cusps of the valves are thrown 

 back and remain thus until the blood is expelled. When the 

 outflow of blood is completed, the cusps are again approxi- 

 mated 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 downwards and together and thus 

 completely prevent any back-flow of blood. 



The prejudicial effect of too great pressure upon these cusps 



