130 HUMAN BIOLOGY 



the pleura. Both surfaces secrete a liquid which enables the lungs 

 to glide over the chest wall without friction. 



187. To determine the amount of enlargement of the 

 chest cavity during inspiration. (Home work.) 



Force the air out of the lungs as completely as possible. 

 Draw a tape or cord around the chest under the armpits, 

 keeping it reasonably tight, and thus measure the girth of 

 the chest. 



1. State what you have done, and record in inches the meas- 



urement thus determined. 



2. Inhale as much air as possible, and again record the chest 



measurement as directed above. 



3. State the difference in the measurements thus obtained. 



4. What is your conclusion, therefore, as to the amount of 



enlargement of the chest cavity ? 



188. How air is taken into the lungs. The chest cavity 

 is not like most boxes inclosed by rigid, immovable walls, 

 for it may be enlarged in its three dimensions ; namely, from 

 side to side, from front to back, and from top to bottom. 

 We shall now consider how this is made possible. A study 

 of the skeleton will show that the ribs are joined to the verte- 

 brae in the back of the chest region and to the breastbone 

 in front in such a way that it is possible to raise and lower the 

 front ends. When the air is forced out of the chest cavity, 

 the front ends of the ribs are lowered and so the breastbone 

 is pulled nearer to the spinal column. As we inspire, the 

 muscles that run from the upper part of the trunk to each 

 x of the ribs contract, and so these bones are pulled upward 

 toward a horizontal position. By this movement the breast- 

 bone is pushed farther away from the spinal column, and the 

 ribs themselves press outward at the sides. In this way the 

 capacity of the chest cavity is increased from side to side, 

 and from front to back. 



