CHAPTEE VII 



MECHANICS OF THE HEART 



CONTENTS. 1. Description of cardiac cycle or revolution. 2. Changes of ex- 

 ternal form, of the internal cavity, of the position and volume of the heart in the 

 different phases of its activity. 3. Mechanism of semilunar valves. 4. Mechanism 

 of auriculo- ventricular valves. 5. Theory of so-called heart-sounds. 6. Variations 

 of pressure within the auricles and ventricles during the cardiac cycle. 7. The 

 diastolic aspiration ; various explanatory hypotheses. 8. Cardiac plethysniograms ; 

 theory of active diastole. 9. Cardiograms ; theory of heart-beats or impulses. 

 10. Other mechanical effects of cardiac activity. 11. Work done by the heart. 

 Bibliography. 



THE continuous circulation of the blood from the arteries to the 

 veins through the capillaries demands, as its first indispensable 

 condition, a mechanism by means of which blood pressure is 

 maintained high in the arteries and low in the veins, so that there 

 is a considerable difference of pressure between the two parts 

 of the vascular system. This mechanism is represented by the 

 heart, which in its rhythmical movements drives as much blood 

 through the aorta and pulmonary artery during systole, as it 

 receives from the venae cavae and pulmonary veins during diastole. 



I. When the movements of the exposed heart are observed in 

 the living animal, a series of phenomena, which are repeated at 

 regular intervals, is witnessed. Each such cycle of movements is 

 known as the cardiac cycle, or revolution. The duration of each 

 cycle is exactly equal to the time interval between any two 

 recognisable arterial pulses. 



This interval may be divided into three periods : in the first 

 is the (normally synchronous) systole of the two auricles ; in the 

 second, the (normally synchronous) systole of the two ventricles ; 

 in the third, the pause or rest of the whole heart. For simplicity's 

 sake, the first may be termed pre-systole ; the second, systole ; the 

 third, peri-systole. The diastole of the auricles coincides with the 

 commencement of systole, the diastole of the ventricles with the 

 commencement of perisystole. 



The words awToK-fj and diao-roXr/ from <ru-a-T^\\iVj contrahere, and 5ia-<TTt\\eiv, 

 distrahere, were first used by Galen. The term peri-systole for the resting' 

 period of the heart as a whole was introduced by Riolan (Encheiridium 



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