HUMAN STRUCTURE AND LIFE-ACTIVITIES 503 



nose are respiratory, and serve the purpose of warming the 

 inrushing air by contact with the warm membranes; and 

 also the membranes and hairs in the nose collect much of 

 the dust from the air which enters. 



The passages connecting those of the nose with the pharynx 

 are called post-nasal. They lie back of the soft palate. 

 Into the same passages open the Eustachian tubes from the 

 ears. Some of the soft tissues in the lining of the post- 

 nasal chamber may enlarge and form adenoids (449), which 

 interfere with free breathing through the nose and cause 

 injurious breathing through the mouth. 



427. Lungs/ (D or L) Examine sheep's lungs obtained from a 

 meat-market. Notice the trachea and the arrangement of its carti- 

 laginous rings, incomplete on the side next to the esophagus. Follow 

 the two branches (bronchi) to the lungs. Try inflating the lungs by 

 forcing air through a large glass or wooden tube inserted into the 

 trachea. Why do the lungs collapse and force out air when pressure is 

 released ? Compare action with that of a rubber bag or balloon. Note 

 that there is air remaining in all parts of the lungs after their complete 

 return to the original size before inflation. In human lungs after 

 ordinary breathing out or expiration of air, the amount of air still 

 left is about 200 cubic inches, and one-half of this can be forced out 

 by "blowing hard" (i.e., forcibly expiring air, which is accomplished 

 by muscular compression of the lungs). This additional discharge 

 of air from sheep's lungs can be demonstrated by pressing upon them 

 with the hands. Of course, the lungs inside an animal cannot be 

 inflated larger than the chest-cavity. 



The delicate outer membrane of the lungs is the pleura, and 

 similar tissue lines the chest-cavity. These membranes are lubri- 

 cated by a secreted liquid which reduces friction between the lungs 

 and the chest-wall during the movements of respiration. Inflam- 

 mation of these membranes, which often occurs in connection with 

 severe "colds," is termed pleurisy. 



Cut into one bronchus and then split it open as you work toward 

 the smaller branches. In this way follow up and lay open a series 

 of successively smaller branches leading out toward the surface of 

 the lung. The smallest branches end in air-chambers. Notice the 

 numerous blood-vessels in the tissues of the lungs. The arteries 

 can be distinguished by having thicker walls than the veins. 



