EVENTS DURING A CARDIAC CYCLE. 



59 



(c) When the ventricles are distended by the inflowing blood, the auriculo- 

 ventricular valves are floated up, partly by the recoil or reflexion of the blood from 

 the ventricular wall, and partly owing to their lighter specific gravity, whereby 

 they easily float into a more or less horizontal position. The valves are also raised 

 to a slight extent by the longitudinal muscular fibres, which pass from the auricles 

 into the cusps of the valve. 



(C) The ventricles now contract, and simultaneously the auricles relax, 

 whereby 



(1) The muscular walls contract forcibly from all sides, and thus diminish the 

 ventricular cavity. (2) The blood is at once pressed against the under surface of 

 the auriculo-ventricular valves, whose curved margins are opposed to each other like 

 teeth, and are pressed hermetically against each other (fig. 32). It is impossible 

 for the blood to push the cusps backwards into the auricle, as the chordae tendinese 

 hold fast their margins and surfaces like a taut sail. The margins of the neigh- 

 bouring cusps are also kept in apposition, as the chordae tendineae from one papillary 

 muscle always pass to the adjoining edges of two cusps. The extent to which the 

 ventricular wall is shortened is compensated by the contraction of the papillary 

 muscle, and also of the large muscular chordae, so that the cusps cannot be pushed 

 into the auricle. When the valves are closed, their surfaces are horizontal, so that, 

 even when the ventricles are contracted to their greatest extent, there remains in 

 the supra-papillary space a small amount of blood which is not expelled (Sandborg 

 and Worm Muller). (3) When the pressure within the ventricles exceeds that in 

 the arteries, the semi-lunar valves are forced open and stretched like a sail across 

 the pocket-like sinus, without, however, being directly applied to the wall of the 

 arteries (pulmonary and aorta), and thus the blood enters the arteries. 



(D) Pause. As soon as the ventricular contraction ends, and the ventricles 

 begin to relax, the semi-lunar valves close (fig. 33). The 

 diastole of the ventricles is followed by the pause. 

 Under normal circumstances, the right and left halves 

 of the heart always contract or relax uniformly and 

 simultaneously. 



Negative Pressure in the Ventricle. Goltz and Gaule found 

 that there was a negative pressure of 23 '5 mm. Hg. (dog) in the 

 interior of the ventricle during a certain phase of the heart's 

 action. This they determined by a maximal and minimal 

 manometer. They surmised that this phase coincided with the 

 diastolic dilatation, for which they assumed a considerable power 

 of aspiration. Moens is of opinion that this negative pressure 

 within the ventricle obtains shortly before the systole has reached 

 Us height, i.e., just before the inner surface of the ventricles and 

 the valves, after the blood is expelled, are nearly in apposition. 

 He explains this aspiration as being due to the formation of an 

 empty space in the ventricle caused by the energetic expulsion 

 of the blood through the aorta and pulmonary artery. 



[Maximum and Minimum Manometer. Into the tube connecting the interior of the ventricle 

 of the heart with the ordinary U-shaped mercury manometer, is introduced the maximum 



Fig. 33. 

 The closed semi-lunar valve of 

 the pulmonary artery seen 

 from below. 



Fig. 34. Fig. 35. 



Gaule's maximum and minimum manometer. 



manometer, which is constructed on the principle of a ball and cup valve (fig. 34), the ball A 

 being kept closed in B by a spring C. To make it a maximum manometer the end A is con- 

 nected with the heart, and B with the mercurial manometer (fig. 35). When a clamp is placed 



