THE HEART 



29 



Mention has been made of valves in the cavities of the heart ; 

 they are found on both sides separating auricle from ventricle, 

 and are known as the right aur iculo - ve , n f ri r.n 1 a r or tricuspid 

 valve, and the left auriculo- ventricular, or mitral valve. Besides 

 these, valves are found in the vessels arising from the ventricles — 

 viz., in the pulmonary artery and the aorta ; these valves, pul- 

 monary and aortic, are known as the semilunar valves. No 

 valves are found guarding the entrance of the vessels (veins) into 

 the auricles. In order to understand the function of these valves, 

 which play such an impor- 

 tant part in the physiology 

 of the heart, it is necessary 

 that we should briefly detail 

 the course which the blood 

 takes from the time it enters 

 the right auricle until it com-X 

 pletes the round of the cir- 

 culation and finds itself at 

 this auricle again. 



Course of the Circulation. 

 — The venous blood from the 

 whole of the body flows into 

 the right auricle by means 

 of the anterior and posterior 

 venae cavae ; it passes from 

 here through the tricuspid 

 valve into the right ventricle ; 

 from the right ventricle it 

 travels to the lungs by means 

 of the pulmonary artery, 

 where, having been exposed 

 to the action of the air and 

 become greatly changed in 

 its gaseous composition, it 

 returns to the heart by means 

 of the pulmonary veins, 

 emptying itself into the left auricle. It now passes through the 

 auriculo-ventricular opening into the left ventricle, and thence 

 into the aorta to be pumped all over the body, being distributed 

 by means of the arteries and capillaries ; it is then collected by 

 the veins, and eventually brought back to the heart to undergo 

 afresh its distribution to the lungs and body (Fig. 11). 



The use of the valves is to allow of and to insure the trans- 

 ference of blood from auricles to ventricles, and from the ventricles 

 to the aorta and pulmonary artery without any chance of 

 regurgitation. This they do in virtue of the fact that they are 



Fig. 10. 



-Diagram of the Circulation 

 through the heart. 



and 2, The venae cavae ; 3, right auricle ; 

 4, right ventricle ; 5, pulmonary 

 artery ; 6, 6, pulmonary veins ; 7, left 

 auricle ; 8, left ventricle ; 9, aorta 

 dividing into anterior or posterior. 

 The arrows represent the direction 

 taken by the blood-stream. 



