180 



RESPIRATION AND EXCRETION 



These must be passed off from the cells, as they are poisons. 

 This is done by the lymph and the blood, taking the waste materials 



to points where they may 

 be excreted or passed out 

 of the body. 



The Pleura. — The 

 lungs are covered with a 

 thin elastic membrane, 

 the pleura. This forms 

 a bag in which the lungs 

 are hung. Between the 

 walls of the bag and the 

 lungs is a space filled with lymph. By this means the lungs are 

 prevented from rubbing against the walls of the chest. 



The Mechanics of Respiration. — In every breath there are two 

 movements, inspiration (taking air in) and expiration (forcing air 

 out). An inspiration is produced by the contraction of muscles 

 between the ribs, together with the contraction of the diaphragm,, 



Respiration of a cell. 



The chest cavity {A) at the time of a full breath : 

 {B), after an expiration. Explain how the chest 

 cavity is made larger. 



Apparatus to show the me- 

 chanics of breathing. 



the muscular wall forming the floor of the chest cavity; this 

 results in pulling the diaphragm down and pulling the ribs up- 

 ward and outward, thus making the space within the chest cavity 



