THE HEART. 



6 7 



of repose and that of action. The period of rest com- 

 mences with the closure of . the sigmoid valve. Its 

 duration varies according to the frequency of the con- 

 tractions. During the whole of it, the cavities fill with 

 blood. The period of action, of which the duration in 

 man is rather more than four-tenths of a second, com- 

 mences with the auricular systole. About one-tenth of a 

 second later, the ventricular systole begins : thereupon 

 the auriculo-ventricular valves close and the blood is 

 suddenly ejected into the aorta and pulmonary artery. 

 At the end of the ventricular systole, which lasts about 

 three and a half tenths of a second, the ventricle sud- 

 denly relaxes. 



The lateral pressure in the auricles is about equal to 

 that of the atmosphere. It rises, however, slightly in 

 auricular systole, attaining its maximum at the com- 

 mencement of ventricular systole. In the ventricles the 

 pressure sinks below that of the atmosphere immediately 

 after the sigmoid valves close ; at the moment of systole 

 it rises above the pressure in the aorta. 



Each action of the heart is accompanied by two sounds. 

 The first is produced by two causes, the muscular con- 

 traction and the sudden tightening of the heart. The 

 second sound is due to the tightening of the aorta and 

 sigmoid valves. 



The filling of the right ventricle may, in the normal 

 state, be attributed to the influence of the elasticity of 

 the lungs, and in its absence to the pressure in the 

 systemic venous system. The filling of the left ventricle 

 is mainly due to the pressure in the pulmonary veins, 

 and to the " aspirating power " of the ventricle itself. It 

 is supposed by Briicke to be aided by the^distension of 

 the coronary arteries. 



It is probable that about 195 grammes of blood are 

 discharged by the left ventricle at each contraction. 

 If the lateral pressure in the aorta were equal to that of 



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