THE HEART 



101 



blood is being forced by the ventricle into the arteries. This 

 is the period of ejection. .There are two brief periods of com- 

 plete closure of the ventricles, i. e., when both cuspid and semi- 

 lunar valves are closed. When the ventricle contracts upon 

 its contained blood and thus raises the pressure, the cuspid 

 valves close readily, but it takes some time for the pressure 

 to become sufficiently high to open the semilunar valves. 

 Similarly when the ventricle relaxes the high pressure in the 

 arteries closes the semilunar valves immediately, but it takes 

 some time for the pressure to fall low enough that the cuspid 



FIG. 2 



Simultaneous tracings from the right auricle and ventricle of the horse: 

 a, a', beginning of cardiac cycle; b, &', rise of pressure due to auricular 

 systole; c', pressure due to ventricular systole; d', oscillations due to 

 inertia; e, e', close of cardiac cycle. (After Chauveau and Marey.) 



valves can open. During the ventricular diastole at the time 

 when the cuspid valves are just opening the blood which has 

 been accumulating in the auricles flows, and to some extent 

 is drawn into the ventricles. This force of slight suction is 

 due to the elastic fibers of the lung, which tend to open the 

 ventricles and also to the elasticity of the heart muscle itself, 

 especially of the auriculoventricular ring. The ventricles can 

 maintain the circulation of the blood for a time at least, 

 unaided by the auricles. The latter form a storehouse for the 

 blood which accumulates during the ventricular systole. The 

 pressure in them in the dog, for instance, seldom rises above 



