360 - Multicellular Animals, Especially Man 



If removed from the body, the lung shrinks 

 down into a relatively small spongy mass. 

 Such a deflated lung may be reinflated 

 through the trachea — in which case the walls 

 become stretched like those of a toy balloon. 

 But if the inflation pressure is released, the 

 elasticity of the stretched pulmonary walls 

 again deflates the lung, expelling most of its 

 content of air. 



The free transit of air to and from the 

 lungs is assured by the fact that the walls of 

 the larynx, trachea, bronchi, and larger bron- 

 chioles are reinforced with cartilage, which 

 prevents the passages from collapsing (Fig. 

 19-3). In the trachea and bronchi the carti- 

 laginous reinforcements take the form of a 

 series of incomplete rings, or bands. But the 

 larynx, which houses the vocal cords, is 

 strengthened by a larger single encasement of 

 cartilage (Fig. 19-3). 



Fach lung is covered externally by a smooth 



epithelium, which is called the pleura; and 

 there is also a pleural lining on the inner 

 surface of the thorax wall (Fig. 19-3). Thus 

 between the two layers of the pleura there 

 exists, at least potentially, a cavity, called 

 the pleural cavity. Normally the pleural cav- 

 ity is practically obliterated by the fact that 

 the outer surface of the lung lies in intimate 

 contact with the inner surface of the thoracic 

 wall. However, if the pleural linings become 

 inflamed, as in pleurisy, fluid tends to accu- 

 mulate in the pleural cavity. 



The pleural cavity has no communication 

 with the outside atmosphere. Above and to 

 the sides it is bounded by the body wall, and 

 below, the pleural cavity is walled off from 

 the abdominal cavity by the diaphragm, a 

 strong dome-shaped sheet of muscle tissue 

 (Fig. 19-3). Pressure in the pleural cavity is 

 generally negative (that is, less than the pres- 

 sure of the outside atmosphere), because the 



Inspiration 



Fig. 19-5. Manner in which the volume of the chest cavity is increased during an 

 inspiration. Each rib is attached to the vertebral column behind, and to the breast- 

 bone in front, and, like the handle of a bucket, moves outward as it is pulled up. 

 In addition, the whole breastbone moves up slightly during an inspiration, so that 

 the distance from vertebral column to breastbone is also increased. Finally, the 

 downward movement of the diaphragm increases the third (vertical) dimension 

 of the chest. (Drawn by E. M. From Gerard, The Body Functions. Permission of 

 John Wiley and Sons, Inc.) 



