270 EXPERIMENTAL GENERAL SCIENCE 



glottis, which shuts down over the opening whenever we swal- 

 low. The lungs are two pinkish, spongy bodies that fill the 

 cavity of the chest with the exception of the part occupied 

 by the heart. They consist of an immense number of tiny 

 sacs with walls of connective tissue lined with mucous mem- 

 brane. In the walls of these sacs are a multitude of capillaries 

 through the walls of which the blood gives up its carbon dioxide 

 and takes on a new supply of oxygen. The lungs are hung 

 loosely in the chest cavity and are surrounded by a mem- 

 brane, the pleura, which is also folded back to form a lining 

 for the thorax. Owing to the pressure of the air within them, 

 the lungs always fill all the space in the thorax. 



229. Breathing. The act of breathing consists in making 

 the thorax larger and thus allowing more air from outside to 

 press in and expand the lungs, after which the impure air is 

 forced out by the weight of the chest walls and other parts. 

 In enlarging the chest cavity, the diaphragm which forms its 

 floor contracts and pressed downward on the contents of the 

 abdomen. At the same time the ribs and breast-bone rise. 

 In ordinary breathing about 30 cubic inches of air are inhaled 

 and exhaled with each breath. There is never a complete 

 change of air in the lungs, however, since the air simply surges 

 to and fro with each breath, but the rapid diffusion of the oxy- 

 gen it contains enables the blood to secure sufficient for the 

 use of the cells. The exchange of gases goes on continuously 

 and not merely at the time the air is taken in. We are never 

 able to expel all the air from the lungs. When we have ex- 

 haled as much air as possible, there is still left about 100 

 cubic inches. By taking a deep breath, the lungs may be made 

 to hold three times this amount. There is some difference in 

 the way men and women breathe. Men use the diaphragm as 

 well as the ribs in breathing, but women breathe mostly by 

 elevating the ribs, a method which the prevailing styles of 

 dress often render necessary. The number of times one 



