338 MANUAL OF PHYSIOLOGY. 



The ordinary shape of the walls of the thorax, when the 

 muscles are not acting, corresponds with the position at the end 

 of gentle expiration ; therefore the resiliency of the muscles, 

 costal cartilages, and other elastic tissues which are stretched 

 during inspiration tends to restore the ribs to the position of 

 expiration. 



The weight of the thorax itself, and the elastic gases in the 

 intestinal tract, which have been compressed by the diaphragm, 

 may also help in expiration. 



After death, when the elasticity of the expiratory muscles is 

 lost, the traction exerted by the lungs on the thorax reduces it 

 below the size its own elastic equilibrium would tend to assume; 

 when, therefore, air is admitted to the pleural cavity by puncture, 

 the thorax expands slightly as the lungs shrink, and the pressure 

 on the pleural surface becomes equal to that within the bronchi. 



In forced expiration, or when the air is used during expiration 

 for any purpose, such as the production of voice, or any blowing 

 movements, a number of muscles are called into action. The 

 only muscles that could be called exclusively special muscles of 

 expiration are the weak triangularis sterni, serratus posticus in- 

 ferior, and parts of the intercostals ; but in all violent and forci- 

 ble expiratory efforts these are aided by the muscles forming the 

 anterior wall of the abdomen, which, associated with the inter- 

 costals and quadratus lumborum, are the most powerful agents 

 in drawing down the thoracic walls. 



FUNCTION OF THE PLEURA. 



From what has been already said it is obvious that by far the 

 greatest amount of movement takes place in the lower part of 

 the thorax, while the capacity of the apex changes but little. 

 The space formed in the chest during inspiration is practically 

 formed between the costal wall and the diaphragm (compare 

 Figs. 148, 149). If the lungs and the walls of the thorax were 

 fused together, without the interposition of serous membranes, 

 the different parts of the lungs would have to follow the move- 

 ments of that part of the thorax to which they are attached. 

 Thus the lower parts of the lung would be much distended dur- 



