A YEAR IN SCIENCE 115 



organs located? What is their position to each other? 

 Notice the trachea with its circular rings of cartilage. 

 Are the rings entire? These are necessary to prevent 

 collapse of the tube. HOAV far down do these rings 

 continue? With a scalpel follow a branch of the 

 trachea into the lungs. How do these branches end? 

 This large amount of branching allows the air to be 

 brought in contact with very small blood vessels, 

 through the walls of which oxygen is absorbed into the 

 blood. 



B. Lungs. How many lobes has each lung? What 

 are their relative sizes ? Note the texture of the lungs. 

 By inserting a glass tube into a branch of one of the 

 bronchi, force air into the lung. What happens? 

 The lungs are the organs chiefly concerned in 

 breathing. 



C. Heart. What is the general shape? The cover- 

 ing about the heart is the pericardium. The heart 

 has four compartments: the upper two, the auricles 

 (right and left) ; the lower two, ventricles (right and 

 left). Which parts have the thickest walls? The 

 Avails are made of muscle with the thick-walled parts 

 doing the pumping. 



Find the blood vessels, superior and inferior vena 

 cava, leading into the right auricle. Open the veins, 

 also the walls of the auricle, and observe the path of 

 the blood into the right ventricle. Note the tricuspid 

 valve that closes this entrance between auricle and 

 ventricle. Also notice the cords bv which this valve 



