894 PHYSIOLOGY 



and prevented from eversion under the strong pressure exerted by the con- 

 tracting ventricle. By the downward pull of the papillary muscles on the 

 valves during the contraction of the ventricles, closure is rendered more 

 complete, the inner surface of the valves being apposed over a considerable 

 area. The action of the valves is aided by the contraction of the fibres 

 surrounding the base of the heart, so- that the auriculo- ventricular orifice 

 is much smaller during systole than during diastole. 



From a purely mechanical standpoint the valves guarding the arterial 

 orifices are much more perfect than those just described, which depend for 

 their efficiency on the proper contraction of the ventricular wall and of the 

 musculi papillares. Each orifice is provided with three valves, each of which 

 is semilunar in shape and attached by its convex borders to the arterial wall, 

 and presents in the middle of its free border a small fibre-cartilaginous 

 nodule, the corpus Arantii, from which fine elastic fibres pass to all parts of 

 the valve. The extreme margin of the valve, the lunula, on each side of the 

 corpus Arantii is very thin, being formed of little more than the endocardium. 

 Whenever the pressure in the arteries is greater than that in the ventricles, 

 these valves are closed, and the thin margins come in contact with similar 

 portions of the adjacent valves, so preventing the reflux of a single drop of 

 blood. The borders of the valves under these circumstances come together 

 in the form of a star composed of three lines at angles of 120, the three 

 corpora Arantii being pressed together at the centre of the star. 



No valves are found at the orifices of the great veins into the auricles, a 

 reflux of blood in this situation during contraction of the heart being 

 limited by the contraction of the muscular rings round the veins, which always 

 accompanies the auricular contraction. 



The heart and the roots of the great vessels lie almost free in a special 

 cavity, the wall of which is formed by a tough fibrous membrane, the 

 pericardium. This is attached below to the central tendon of the diaphragm, 

 and above to the arterial trunks. It is lined by a layer of endothelium 

 continuous with a similar layer covering the surface of the heart. The 

 two surfaces are kept continually moist by the pericardial fluid, so that the 

 heart can move freely within the pericardium without friction. One of the 

 chief functions of the pericardium appears to be to check an excessive 

 dilatation of the heart during conditions attended by a great rise of venous 

 pressure. 



THE SEQUENCE OF EVENTS IN THE CARDIAC CYCLE 



On opening the chest of an anaesthetised animal, while artificial respira- 

 tion is maintained, the heart is seen contracting rhythmically within the peri- 

 cardium. On incising this sac its restraining power on the dilatation of the 

 heart is shown by the fact that during diastole the wall of the heart bulges 

 through the opening, and the increased diastolic filling, consequent on the 

 removal of this restraining influence, is at once apparent, if in any way the 

 frequency of the contractions of the heart be diminished so as to prolong the 

 diastolic period. 



