THE HEART 97 



is most likely due to a vibration of the auriculoventricular 

 valves, early in diastole, when the inflow of blood from the 

 auricles distends the ventricles sharply and suddenly, momen- 

 tarily throwing the valves into a position of closure. 



The average rate of heart beat in an adult man is about 

 72 a minute, and is somewhat faster in women. It varies, 

 however, so that in some individuals it may be 40 or 100 a 

 minute. Shortly before and after birth it averages from 120 

 to 140. During extreme age its frequency is increased. It 

 is influenced by many conditions of bodily health and environ- 

 ment, such as sleep, position, temperature, meals, and emotions. 

 Exercise may increase it to 200 or more. 



The auricular systole is rapid, and forces the blood into the 

 still quiescent ventricles. On completion of the auricular 

 systole the auricles expand and remain quiet during the systole 

 of the ventricles, which begins the moment the auricles cease 

 contracting. The ventricular systole is more forcible than 

 that of the auricles, and is sustained for a greater length of 

 time. During ventricular systole the blood is forced into the 

 arteries. At the close of the ventricular systole the ventricles 

 dilate. The auricles do not take up the work again at once, 

 but there is a period during which the entire heart is in repose. 

 After this a new cycle is begun. If we assume the average 

 number of heart beats to be 72 a minute, each cardiac cycle 

 occupies 0.8 of a second. The contraction of the auricles lasts 

 0.1 of a second; that of the ventricles, 0.3 of a second; and the 

 repose of the entire heart, 0.4 of a second. If the heart rate 

 be increased, the ventricular systole remains about 0.3 of a 

 second, and the increase in rate is made mainly at the expense 

 of the time occupied by the diastole. 



It is the function of the heart to force the blood in one direc- 

 tion only, and this is effected by means of the valves. As the 

 ventricle fills, the auriculoventricular valves are floated up 

 from the sides of the ventricle in such a manner that their 

 edges are brought into contact. As the ventricle contracts 

 more forcibly, pressure is brought upon the valves, so that 

 not only are the edges in contact, but also portions of the sur- 

 faces of the cusps. These valves are of considerable area, 

 and are held in position by the chorda? tendince, which arise 

 from the papillary muscles, so that eversion of the valve into 

 7 



