27 6 TEXT-BOOK OF PHYSIOLOGY. 



S eXm The heart-beat is more rapid in females than in males. 

 Thus while the average beat in males is 72, in females it is usually 

 8 or 10 beats more. 



Posture. Independent of muscle efforts the rate of the beat is 

 influenced by posture. It has been found that when the body is 

 changed from the lying to the sitting and to the standing position, 

 the heart will vary as follows from 66 to 71 to 8 1 on the average. 



Exercise and digestion also temporarily increase the number of 

 beats. 



The Action of the Valves. As previously stated, the forward 

 movement of the blood is permitted and regurgitation prevented by 

 the alternate action of the auriculo-ventricular and the semilunar 

 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 ventricular 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 accomplished 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\:or behind 

 them are the pouches of ValsalvaVontaining 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 appo- 

 sition. By this means the orifices of the pulmonary 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 (pntricular systole [the frelaxeb!) auricles have been 

 filling with blood. \Vith the ventricular relaxation Ibis volunie, 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 ven cavae and pulmonary veins. WhetherJie 

 j vgntricles | exert a suction power at foe mnmppt nf their relaxations 

 an undecided question. A steady stream of blood into the auricles 

 ano^ 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 cur- 

 rents 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. 



With the cessation of the auricular systole the ventricular systole 

 begins. If the blood is not to be returned to the auricles at this 



