THE RESPIRATORY SYSTEM 153 



anterior, middle, and posterior mediastina, which have 

 their several anatomic boundaries. 



The ribs are elevated during inspiration and depressed 

 during expiration; the chest is consequently expanded 

 when the air is taken in and contracted when it is given 

 out. 



THE DIAPHRAGM 



The diaphragm is a firm, musculofibrous septum, situ- 

 ated transversely across the cavity of the trunk, at the 

 lower margin of the bony walls of the chest. It is convex 

 on its upper, and concave on its lower, surface. By the 

 contraction of the muscle-fibers in inspiration, the plane of 

 the diaphragm becomes flattened and the lungs swell and 

 fill the enlarged cavity of the chest, the air rushing in, on 

 the principle of common air-pressure. 



REVIEW QUESTIONS 



What constitutes the respiratory system? 

 What is respiration? 

 Into how many acts is it divided? 

 What takes place in respiration? 

 How are the lungs divided? 

 What is the weight of the lungs? 

 Where are they situated? 

 What membrane surrounds them? 

 Which lung is the larger? 

 What is the trachea? 

 How do the bronchial tubes divide? 

 What are the volumes of air called? 



State the average volume of air taken in at an ordinary inspiration. 

 Define reserve air, complementary air, and supplementary air. 

 What are the constituents of the inspired air, and also of the expired 

 air? 



What accessory organs are involved in respiration? 



Describe the pleura. 



What is the mediastinum? 



Describe the diaphragm. 



What nerve controls respiration? 



