52 A Manual of Veterinary Physiology. 



time the blood was being collected in the auricles ; as the 

 ventricles fill their valves float up into position, the chordre 

 tending are rendered tense, and the ventricle contracts 

 by a peculiar wringing movement produced by the arrange- 

 ment of its fibres. The blood now presses against the 

 auriculo-ventricular valves, closing them more firmly, and 

 as the walls contract the imprisoned blood is forced up 

 into the aorta and pulmonary artery, pressing open the 

 valves situated here owing to the difference in pressure 

 between the blood in the ventricles and that already in 

 these vessels. During this process of ventricular contraction 

 a sound is produced, occasioned by the muscular contrac- 

 tion of the walls, and probably by the vibration of the 

 tricuspid and mitral valves. Neither ventricle completely 

 empties itself during contraction (Colin). 



The blood now rushes into the aorta and pulmonary 

 artery, but soon the pressure in the ventricle becomes nega- 

 tive, viz., lower than that in these vessels, and at the same 

 time the elastic resistance of these arteries being brought 

 into play, the blood has a tendency to regurgitate towards 

 the ventricles and by so doing closes firmly the semi-lunar 

 valves, a sharp sound being produced at this moment by 

 their closure. While the ventricle is contracting the auricle 

 is dilating, and so soon as the semi-lunar valves are closed 

 there is a pause in the movement of the heart, during 

 which period the auricles and ventricles are relaxed and 

 blood is flowing into them ; at the end of the pause the 

 auricles contract and the whole process is repeated. 



We have thus the contraction of the auricles, the con- 

 traction of the ventricles, and the pause. The periods 

 these occupy have been determined for the horse, by 

 Chauveau and Marey, by means of a cardiac sound. The 

 value of the periods is as follows: auricular systole, two- 

 tenths of a second, ventricular systole, four-tenths, and 

 pause, four-tenths of a second. The duration of the ventri- 

 cular systole always remains the same, viz . tour-tenths of a 

 second, no matter how fast, the heart, is beating, so that the 

 frequency depends not, on the time occupied in the con- 



