Chap. IX.] 



THE VASCULAR SYSTEM. 



109 



The contractions of the heart are rhythmical; that is to say, 

 they occur in a certain order. First, there is a simultaneous 

 contraction of the walls of both auricles; immediately following 

 this, a simultaneous contraction of both ventricles; then comes 

 a pause, or period of rest, after which the auricles and ven- 

 tricles contract again in the same order as before, and their 

 contractions are followed by the same pause as before. The 

 state of contraction of the 

 heart is called the systole ; 

 the state of relaxation and 

 dilatation, its diastole. 



If the chest of an ani- 

 mal be opened and arti- 

 ficial respiration kept up, 

 the heart may be watched 

 beating, and a complete 

 beat of the whole heart 

 may be observed to take 

 place as follows : — 



The great veins are 

 seen, while full of blood, 

 to contract in the neigh- 

 bourhood of the heart, 

 the wave of contraction 

 running on towards the 

 auricles, increasing in in- 

 tensity as it goes. Arrived at the auricles, which are now full 

 of blood, the wave of contraction passes on to them, and they 

 contract suddenly and quickl3^ During this contraction, the 

 walls of the auricles press towards the auriculo-ventricular ori- 

 fices, and the blood passes over the tricuspid and mitral valves 

 into the ventricles. The ventricles fill rapidly, and as soon as 

 the auricular contraction is over, they in turn are seen to con- 

 tract, their walls becoming very tense and hard; the apex is tilted 

 upwards, and the heart twists somewhat on its own axis. Dur- 

 ing the ventricular contraction the blood in the ventricles is 

 forced through the semilunar valves into the arteries, which are 

 seen to elongate and expand as the blood is pumped into them. 



The work of the auricles and ventricles is very unequal. All 

 the auricles have to do is to pump the blood into the ventricles, 



Fig. 84. — Section of Heart at Level of 

 Valves. P, pulmonary artery, with Haps of 

 semilunar valve open; A, aorta, with flaps of 

 semilunar valve open ; M, closed mitral valve ; 

 T, closed tricuspid valve. 



