THE HEART 569 



of Stokes-Adajns disease (heart block). This condition is characterized by a slow pulse, a 

 tendency to syncopal or epileptiform seizures, and the fact that while the cardiac auricles 

 beat at a normal rate, the ventricles contract much less frequently. The existence of a bursa 

 in relation with the bundle suggests the possibility of a bursitis, in view of which these cardiac 

 symptoms may be the result of acute rheumatism or other febrile diseases. 



Vessels and Nerves. The arteries supplying the heart are the right and left coronary from 

 the aorta. 



The veins terminate in the right auricle, and will be described with the general venous system. 



The lymphatics end in the thoracic and right lymphatic ducts. 



The nerves are derived from the superficial and deep cardiac plexuses, and from these plexuses 

 obtain fibres of the vagus, spinal accessory, and sympathetic. The superficial cardiac plexus lies 

 under the arch of the aorta. The deep cardiac plexus is in front of the tracheal bifurcation. 

 The nerves from the plexuses are freely distributed both on the surface and in the substance of 

 the heart, the separate filaments being furnished with small ganglia. A special system of gan- 

 glion cells and nerve fibres has been found 1 in the auriculoventricular bundle. 



The Cardiac Cycle and the Action of the Valves. By the contractions and pumping 

 action of the heart the blood is forced through the arteries, capillaries, and veins 

 of the systemic and pulmonic vascular systems. Normally, these contractions are 

 rhythmic in character and occur at the rate of about seventy per minute. Each 

 period of activity is followed by a period of rest, and during these two periods 

 certain events take place in the various parts of the heart in regular sequential 

 order. The period included between the occurrence of any one of these events 

 and the recurrence of the same event constitutes a cardiac cycle, or cardiac revolution. 



The cardiac cycle may be conveniently divided into three phases, which succeed 

 one another, as follows: (1) A short, practically simultaneous contraction of both 

 auricles, termed the auricular systole, followed, after a slight pause, by (2) a simul- 

 taneous, but more prolonged, contraction of both ventricles, named the ventricular 

 systole, and (3) a period of rest during which the whole heart is relaxed, i. e., in a 

 state of diastole. The contraction process begins at the venous openings or in an 

 area in the right auricle between the venae cavae, whence it spreads, in the form 

 of a wave, over the auricles and then to and over the ventricles. The quick 

 contraction of the auricles forces the blood contained in these chambers, through 

 the auriculoventricular openings into the relaxed ventricles, which become 

 fully distended. The contraction of the ventricles follows almost immediately. 

 There ensues a rapid compression of their contained mass of blood and a 

 relatively high pressure is thus developed which occasions the forcible closure 

 of the auriculoventricular valves. These are prevented from being everted into 

 the auricular cavities by their attachment to the papillary muscles through the 

 intermediation of the chordae tendineae. The diminution in size of the ventricular 

 cavities might be followed by a slackening of these tendinous cords were it not 

 for the compensating effect of the active shortening of the papillary muscles which 

 takes place shortly after the onset of the contraction of the general ventricular 

 musculature. The pressure in the ventricles soon rises above that in the pul- 

 monary artery and aorta. At that moment the valves at the orifices of these vessels 

 are forced open and the blood is driven, by a sustained contraction, from the right 

 ventricle into the pulmonary artery and from the left ventricle into the aorta. 

 As soon as the ventricular systole ceases and the pressure in the pulmonary artery 

 and aorta exceeds that in the ventricles the pulmonary and aortic valves close, thus 

 preventing a regurgitation of the blood into the ventricles. While the ventricle? 

 are contracting blood is flowing from the veins into the auricles, where it accumu- 

 lates and distends them. As the ventricles relax, the pressure of the blood in the 

 auricles opens the auriculoventricular valves and blood now flows passively from 

 the veins into the auricles and from these into the ventricles. The rapid accumu- 

 lation of the blood in the ventricles leads to a floating up of the auriculoventricular 



1 J. Gordon Wilson, Proceedings of the Royal Society, B., 1909, vol. Ixxxi. 



