938 PHYSIOLOGY 



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 fibro- 

 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 

 pcrieanlium. 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. ;md the increased diastolic filling, consequent 

 on tin 4 removal of this rest raining influence, is at once apparent, if in any 

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

 prolong the diastolie period. 



Each heat of the heart he'Jns by a simultaneous contraction of both 



