316 KBSPIEATION 



smaller branches called bronchial tubes, and finally end 

 in thin-walled sacs called pulmcmary sacs. The bronchial 

 tubes and the sacs are all inclosed in two baglike bodies 

 covered with a membrane, and located in the chest cavity 

 one on each side of the heart. These two bodies, the 

 framework of which consists of the branches of the trachea, 

 are what we designate by the name of lungs, and the pul- 

 monary sacs of these lungs are the ultimate destination 

 of the inhaled air. The structure of the windpipe and 

 lungs of man are practically identical with those of all 

 other mammals, and the dissection of the chest cavity of 

 such a mammal (rat, cat, or rabbit) gives one a very clear 

 idea as to the structure of our own organs, and their posi- 

 tion with relation to the other organs of the body. 



Position and structure of the lungs. — (See Ex. LVIII.) 

 If we remove the skin from the chest and throat of a mam- 

 mal the windpipe and the ribs will be laid bare. Between 

 the ribs and connecting them are layers of muscle (the 

 intercostals), and on moving the point of the breastbone 

 up and down the layers will be seen to give a peculiar 

 movement to the ribs. We can produce this same move- 

 ment by inserting a glass tube in the entrance of the 

 trachea and inflating the lungs with the breath. As we 

 do this we shall also note a downward movement of a 

 membrane which separates the chest cavity from the 

 abdominal cavity. This membrane is the diaphragm, and, 

 together with the ribs, it performs an important action 

 in breathing movements. The relation of these boundary 

 walls of the chest cavity are shown in Figure 144. 



Cutting the ribs on one side close to the breastbone, 

 and pressing them back, the lungs and heart which fill the 

 cavity are seen. The lungs appear as two pinkish bags, 



