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TEXT-BOOK OF PHYSIOLOGY 



ventricular valves, d, a large portion of the volume of the blood in the auricles 

 flows quickly into the ventricles during the early part of the diastole, nearly, 

 if not entirely refilling them, a procedure occupying a period of time ap- 

 proximately that required for the systolic discharge. In some experiments 

 the portion of the curve, recording the increase of volume does not reach the 

 abscissa but runs off parallel with it. In other experiments it gradually 

 approximates the abscissa or shows a slight fall at the time of the succeeding 

 auricular contraction. According to Henderson blood has ceased to flow 

 from the auricles into the ventricles during the foregoing period and the 

 contraction of the auricles adds but little to the volume of the blood already 

 in the ventricles. Other investigators attach more importance to this 

 event. 



The portion of the volume curve between the apex and the beginning 

 of the succeeding ventricular systole, embracing the entire diastolic period, 

 has been subdivided into two portions, viz. : (a) from the apex to the be- 

 ginning of the horizontal portion, corresponding to the period of relaxation 

 and refilling the diastole proper and (6) the period of rest or the diastasis. 

 This latter period is of rather short duration in the normal rate of heart 

 beat. It lengthens if the rate decreases, and shortens if the rate increases. 



The Action of the Valves During the Cycle. As previously stated, 

 the forward movement of the blood is permitted and regurgitation prevented 

 by the alternate action of the semilunar and the auriculo-ventricular valves. 

 As a point of departure for a consideration of the action of the valves and 

 their relation to the systole and diastole of the heart, the close of the ventricu- 

 lar systole may be conveniently selected. 



At this moment, if the blood is not to be returned to the ventricles, the 

 semilunar valves must be instantly and completely closed. This is accom- 

 plished in the following manner: During the outflow of blood from the 

 ventricles the valves are pushed outward toward the walls of the vessels, 

 though not coming into contact with them, for behind them are the pouches 

 of Valsalva, containing blood, continuous with and under the same pressure 

 as that in the vessels themselves. With the cessation of the outflow and the 

 beginning of the relaxation the pressure of the blood behind the valves 

 suddenly forces them inward until their free edges, including the lunulae, 

 come into complete apposition. By this means the orifices of the pulmonic 

 artery and aorta are securely closed and a return flow prevented. Reversal 

 of the valves is prevented by their mode of attachment to the fibrous rings of 

 the orifices. 



During the ventricular systole the relaxed auricles have been filling with 

 blood. With the ventricular relaxation this volume, or its equivalent, flows 

 readily into the empty and easily distensible ventricles, its place being taken 

 by an additional volume of blood flowing from the venae cavae and pulnf6nic 

 veins. Whether the ventricles exert a suction power at the moment of 

 their relaxation is an undecided question. A steady stream of blood ii4 

 the auricles and ventricles continues throughout the entire period of rest until 

 both cavities are filled. The tricuspid and bicuspid valves which hang down 

 into the ventricular cavities are now floated up by currents of blood welling 

 up behind them until they are nearly closed. The auricles now contract, 

 forcing their contained volumes, or at least the larger portions of them, into 

 the ventricles, which become fully distended. 



