THE HEART. 625 



auricle, by its muscular apparatus, contracts, and throws 

 the blood through the ostium venosurn, into the right ven- 

 tricle. This ventricle now contracts and propels the blood 

 into the pulmonary artery ; the tricuspid valve preventing 

 its return into the auricle. The pulmonary artery con- 

 ducts the blood to the air cells of the lungs, where it is 

 changed from venous into arterial,, as already explained 

 under the head of respiration, and its return to the ven- 

 tricle is prevented by the semilunar valves placed at the 

 mouth of the artery. From the lungs it is carried by the 

 four pulmonary veins to the left side of the heart, into the 

 left auricle,, thus completing a circle from the right to the 

 left auricle, called the pulmonic or lesser circulation. 



The left auricle now contracts and throws the blood 

 through the ostium arteriosum, into the left ventricle, which 

 in turn contracts and propels it into the aorta ; the mitral 

 valve between the auricle and ventricle, and the semilunar 

 valves at the mouth of the aorta, preventing any regurgi- 

 tation. From the aorta it passes to every part of the sys- 

 tem, returning by the vena cava to the right side of the 

 heart, and thus completing another circle called the systemic 

 or greater circulation. 



In the passage of the blood through the heart, two dis- 

 tinct sounds are heard, known as the sounds of the heart. 

 The first sound is dull, prolonged, and corresponds with the 

 contraction of the ventricles, and the impulse of the heart 

 against the ribs. The second sound, compared to a click, is 

 sharp, clear, and quick, and corresponds to the diastole of 

 the heart. The two sounds embrace one arterial pulsation. 

 The two auricles contract synchronously, so likewise the 

 two ventricles. The first sound, it is believed, results from 

 the contraction of the muscular fibres of the ventricles, the 

 impulse of the heart's apex against the ribs, and the rush of 

 the blood through the aorta and pulmonary artery. The 

 second sound is thought to arise from the sudden filling and 

 quick closure of the semilunar valves, by the reflux of the 

 blood during the diastole of the ventricles, and the recoil of 

 the elastic coat of the arteries ; other opinions are enter- 

 40 



