1008 PHYSIOLOGY 



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 peri- 

 cardium 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 

 respiration is maintained, the heart is seen contracting rhythmically 

 within the pericardium. 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 con- 

 tractions of the heart be diminished so as to prolong the diastolic 

 period. 



Each beat of the heart begins by a simultaneous contraction 

 of both auricles associated with a retraction of the auricular 

 appendages, which become pale and bloodless. After a pause of 

 not more than a tenth of a second the contraction of the auricles 

 is followed by that of the ventricles, and blood is thrown out 

 into the large arteries. The contraction of the auricles lasts about 

 a tenth of a second, that of the ventricles about three- tenths of a 

 second. The period of relaxation or diastole lasts about four-tenths 

 of a second. During this cycle of changes the following events are 

 taking place within the heart : 



In the diastolic period the aortic valves are closed and the arterial 

 system is opened only towards the capillaries. In consequence of 

 the high pressure established within the arteries by the previous 

 heart-beats the blood flows steadily through the arterioles, capil- 

 laries, and veins into the right heart, and similarly the pressure 

 in the pulmonary artery causes a partial emptying of this vessel with 

 its branches through the pulmonary capillaries into the left heart. 

 The flow into the heart is assisted by the elastic retraction of the lungs, 

 which causes a negative pressure in the structures between them and 

 the chest wall, so that the blood is sucked from the other parts of the 

 body towards the thorax. During diastole there is a continuous flow 

 of blood from veins into auricles and from auricles into ventricles, and 

 as the walls of both these cavities are relaxed there is no impediment 

 to the inflow of the blood until the dilating heart begins to stretch 

 the pericardium. 



Under normal circumstances the diastole comes to an end before 



