340 A YEAR IN SCIENCE 



Action of heart. The left and right sides of the heart 

 Avork in unison. However, for convenience, we will 

 follow the flow of the blood through one side of the 

 heart and then through the other. Blood enters the 

 right auricle through two large veins. One, the 

 superior vena, brings blood from the upper part of the 

 body; the other, the inferior vena, carries blood from, 

 the lower part of the body. From the right auricle 

 the blood enters the right ventricle. Then it is forced 

 into a large artery, the pulmonary artery, which takes 

 it to the lungs where the blood receives a new supply of 

 oxygen and loses carbon dioxide. It returns to the 

 left auricle through four pulmonary veins, and then 

 enters the left ventricle from which it is forced to all 

 parts of the body through the aorta. 

 . To the cells of the body it furnishes food and oxygen. 

 Into it they return waste products, carbon dioxide, 

 water, and nitrogenous substances. With the exception 

 of carbon dioxide, most of these waste products are 

 removed from the veins before the blood again returns 

 to the heart through the vena cavas. 



There are two sets of valves in the heart. One set, 

 already referred to, prevents the blood from the ven- 

 tricles returning to the auricles. The other valves are 

 found one in each artery. These prevent the blood in 

 the arteries from returning to the ventricles. 



Beat. A beat of the heart is the contraction of the 

 walls of the auricles and of the ventricles. The two 

 auricles contract at the same time; then the two ven- 



