Fig. 18-3. In breathing the thoracic cavity increases and decreases in size by alter- 

 nate raising and lowering of the ribs, together with a corresponding elevation 

 and depression of the diaphragm. This can be shown experimentally by placing 

 the lungs and trachea of a dog in a bell jar and covering the open end with a 

 rubber membrane. By pushing up and down on the membrane, simulating the 

 movements of the diaphragm, air passes in and out of the lungs. 



