THE CIRCULATION OF THE BLOOD AND LYMPH 79 



freely in the entering current of blood, are floated up as the 

 intraventricular pressure begins to rise, so that, in the first 

 moment of the sudden and powerful ventricular systole, the free 

 edges of their segments come together, and the auriculo-ven- 

 tricular orifices are completely closed (Fig. 85, p. 190). In the 

 measure in which the pressure in the contracting ventricles 

 increases, the contact of the valvular segments TW$6nies closer 

 and more extensive ; and their tendency to belly into the auricles 

 is opposed by the pull of the chordae tendineae, whose slender 

 cords, inserted into the valves from border to base, are kept 

 taut, in spite of the shortening of the ventricles by the contrac- 

 tion of the papillary muscles. The arrangement and connections 

 of the muscular fibres of the heart are such that during the 

 auricular systole the auriculo-ventricular groove moves towards 

 the base of the heart, while during the systole of the ventricles 

 it moves towards the apex, which constitutes a relatively fixed 

 point on account of the mutual action of the numerous fibres 

 which converge here and constitute the " whorl." The line joining 

 the apex and the origin of the aorta does not shorten when the 

 ventricles contract, but all parts of the heart are drawn towards 

 this line. The apex is, therefore, pushed forwards, while the rest 

 of the ventricular surface is being drawn inwards. During the 

 systole, the ventricles change their shape in such a way that their 

 combined cross-section which in the relaxed state is a rough 

 ellipse with the major axis from right to left becomes approxi- 

 mately circular, and they then form a right circular cone. As 

 soon as the pressure of the blood within the contracting ventricles 

 exceeds that in the aorta and pulmonary artery respectively, the 

 semilunar valves, which at the beginning of the ventricular 

 systole are closed, yield to the pressure, and blood is driven from 

 the ventricles into these arteries. 



The ventricles are more or less completely emptied during the 

 contraction, which seems still to be maintained for a short time 

 after the blood has ceased to pass out. The contraction is fol- 

 lowed by sudden relaxation. The intraventricular pressure falls. 

 The lunules of the semilunar valves slap together under the 

 weight of the blood as it attempts to regurgitate, the corpora 

 Arantii seal up the central chink, and the aorta and pulmonary 

 artery are thus cut off from the heart. Then follows an interval 

 during which the whole heart is at rest, namely, the interval 

 between the end of the relaxation of the ventricles and the 

 beginning of the systole of the auricles. This constitutes the 

 pause. The whole series of events is called a cardiac cycle or 

 revolution (see Practical Exercises, p. 186). 



It will be easily understood that the time occupied by any 

 one of the events of the cardiac cycle is not constant, for the 



