58 ELEMENTARY PHYSIOLOGY. 



We will now trace and learn the circu- 



of^Biood. lation or course of the bl od in the 

 body. This diagram* in the center of 

 this plate will help us clearly to understand it. Here 

 the heart (32) is laid open, showing its inner cham- 

 bers and the valves between them. This large blood- 

 vessel (16) is one of two great veins which empty 

 the impure blood from the body into the right 

 auricle of the heart. It is called the ascending vena 

 cava, because it brings the blood from parts below 

 the heart. Here (15) is the other of these large 

 veins the descending vena cava, bringing the 

 impure blood from the upper parts. A valve pre- 

 vents the blood from going back from the right ven- 

 tricle into the auricle above. Here (21) is the 

 pulmonary artery, which carries the blood from the 

 right ventricle to the lungs. The blood is driven 

 through this artery by the contraction of the muscle 

 of the ventricle. Coming back from the lungs to the 

 heart, the blood flows through this (23) pulmonary 

 vein, which empties into the left auricle. From here 

 it goes through a valve into the left ventricle. Now 

 notice the thick muscle of the left ventricle. When 

 this strong muscle contracts, the blood is forced out 

 through this (24) great artery the aorta, which 

 branches out into many arteries and then into capil- 

 laries all through the body. It will be well for 

 you to learn and trace the course of the blood in this 

 way: Coming impure from the body, it flows into 



* See plate showing "Formation and Circulation of the Blood." 



