KESPIKATION 33 



animals were taken from the water and left exposed 

 to the air, it would die for lack of oxygen, although there 

 is such a large proportion of oxygen in the air, because 

 oxygen can pass through a membrane only when the 

 membrane is moist. In these animals, the membrane 

 is situated practically on the outside of the body. It 

 is kept moist by the water in which they live, but it 

 would soon become dry and hard if exposed to the air. 

 In animals not surrounded by water the membrane is 

 protected by being situated in a moist chamber so far 

 removed from the surface that it can not dry out. In 

 insects it lines branching tubes; in other land animals 

 it is developed into special organs called lungs. 



Respiratory Organs in Land Animals. — Each lung 

 is like a bunch of hollow grapes, with very thin skins. 

 Each grape-like hollow is filled with air and is called 

 an air cell. The thin skin is an elastic membrane in 

 which is a network of tiny blood vessels. The air inside 

 the air cells is thus separated from the blood in the 

 blood vessels by a thin, moist membrane which can not 

 grow dry because the lungs are situated in a moist 

 chamber. Oxygen from the air inside each air cell 

 can pass into the blood through the lung membrane 

 as easily as it passes from water into the blood of the 

 fish through the gill surface. 



Mechanism for Bringing Air in Contact with the 

 Membrane. — Lungs are so far removed from the surface 

 of the body that a special mechanism is necessary to 

 bring the air into the air cells in contact with the lung 

 membrane. This mechanism is beautifully developed 

 in human beings but it is not peculiar to them. It 



