134 STUDIES IN PHYSIOLOGY 



blood that is forced into it. Hence, when the ventricle begins 

 to relax, the blood tends to rush back into this chamber from 

 the pulmonary artery. To prevent this three sem-i-lu'nar 

 valves (Latin semi == half -f luna = moon) are placed at the 

 opening of the artery. Each valve is shaped like a watch 

 pocket. The three open outward from the heart, but as soon 

 as the ventricle begins to relax, the blood fills up the pockets, 

 and the three valves, by meeting in the middle of the artery, 

 keep the blood from returning to the ventricle (Figs. 49 and 

 50). 



The Blood Vessels connected with the Left Heart. : After 

 receiving oxygen in the lungs, the scarlet blood is brought to 

 the left auricle by four pulmonary veins (Fig. 49). It is then 

 forced into the left ventricle and out into the a-or'ta, which is 

 the largest artery of the body. Branches of this aorta supply 

 blood to every part of the body from the crown of the head 

 to the tips of the toes (Fig. 56). At the opening of the aorta 

 are three semilunar valves, which work just like those in 

 the right heart. 



The Beat of the Heart. If one etherizes a frog, and then 

 opens the chest cavity, one can watch the regular beating 

 of the heart. First the two auricles contract at the same 

 time, becoming paler in color, thus showing that the blood 

 has been forced down into the ventricle. As soon as the 

 auricles have ceased to contract, the apex of the heart (con- 

 taining the ventricle) begins its work, driving the blood out 

 into the pulmonary artery and the aorta. Meanwhile, the 

 auricles have been relaxing and filling with blood. When the 

 ventricle has emptied itself of blood, it also relaxes. If one 

 feels of the heart at this time, it is found to be soft and 

 flabby, while during contraction it is hard. 



The action of the human heart is much like that described 

 for the frog. Each heart beat, therefore, consists of a contrac- 

 tion of the two auricles, followed by a contraction of the ventri- 

 cles; then comes the relaxation of the muscular walls and a 

 pause , in which the chambers arejilled with blood. 



