COURSE OF CIRCULATION IN MAMMALS 



149 



each ventricle, and thus the cavity of each comes to be equivalent 

 to a sphere surrounded by muscle. 



When the intraventricular pressure rises the least bit above that 

 in the pulmonary artery and aorta respectively, the semilunar valves 



A B 



CD 



High and low 



Intra-Ventricular 



pressure 



0.5 



0.6 



0.7 



0.8 sec, 



FIG. 56. THE TIME RELATIONS OF THE IMPULSE, INTRA-AURICULAR AND VENTRIC- 

 ULAR, AND ARTERIAL PRESSURE CURVES. (Modified from Htirthle; L.H., from 

 Allchin's "Manual of Medicine.") 



A, The beginning of ventricular systole; B, the opening of the semilunar valves; 

 A-B, period of rising tension ; C, the closure of the semilunar valves ; A-C, period 

 of output (systole); D, the beginning of the dicrotic wave on the pulse curve; 

 C-E, period of diastole ; E, the beginning of the next cardiac cycle. The dura- 

 tion of the first and second sound of the heart is marked below the intra-auric- 

 ular pressure curve. The pressures are given in mm. Hg. The waves on 

 the plateau of the ventricular curve are due to elastic vibrations and are partly 

 instrumental ; the first fall in the curve of auricular pressure is produced by the 

 systolic output of blood ; the second auricular fall is due to ventricular diastole. 

 The high-pressure pulse is anacrotic. 



quietly open (B, Fig. 56) and the blood is expelled. The elastic vessels 

 are in their turn expanded by the expulsive force of the heart, so as 

 to receive the blood. 



The outflow of blood from the ventricles is rapid at first. It 

 becomes slower as the arteries become distended and the pressure of 



