374 CIRCULATION AND RESPIRATION 



lower chambers are the right and left ventricles. The 

 auricles force the blood into the ventricles ; then the ven- 

 tricles exert the pressure that drives the blood to the 

 body and lungs. The ventricles have much thicker walls 

 than the auricles, because they have so much harder 

 work to do; and the left ventricle, which forces the 

 blood through the body circulation, has walls much 

 thicker than the right ventricle, which sends the blood 

 through the lungs. 



The heart has 4 valves which permit its outlets to be opened and 

 closed. The valves are made of tough connective tissue, and are 

 strengthened by cords fastening them to the walls of the heart. Blood 

 pressing on one side of the valves makes them open, but when the 

 pressure is on the other side, they come together, and prevent the 

 blood from flowing backward. We are accustomed to think of the 

 heart as a pump; we must rather think of it as 4 pumps. Two valves 

 permit the blood to pass from the auricles into the ventricles, and two 

 more guard the openings to the two great arteries that carry the blood 

 to the lungs and body. The " beating" of the heart is due to the 

 pressing of its small end against the chest walls; this takes place at 

 each contraction of the ventricles. The heart of a child in its first 

 year beats about 120 times each minute. In the fourteenth year the 

 number of beats is about 85; in the adult, about 70 or 80. 



376. Arteries and Veins. The tubes that carry blood 

 from the heart are called arteries; those bringing blood 

 to the heart are veins. The artery that carries blood 

 from the left ventricle (Fig. 285) is called the aorta; this 

 artery and its branches supply the body circulation. The 

 pulmonary artery, coming from the right ventricle, takes 

 blood to the lungs. 



Arteries are made of 3 layers of tissue; the middle one 

 contains muscular fibers. Thus, while the arteries are 



