346 THE ANIMALS AND MAN 



To accomplish an inspiration, the thoracic or pleural cavity 

 must be enlarged. The pleural cavity is completely en- 

 closed, being bounded on the front by the sternum, on the 

 sides by the ribs with their intercostal muscles, and on the 

 back by the backbone. The diaphragm forms the floor of 

 the cavity. This is a dome-shaped sheet of muscle (figs. 

 159, D and 167, D) convex upward. In ordinary breathing, 

 the muscles between the ribs, called intercostal muscles (fig. 

 167, I), the muscles between the backbone and ribs, called 

 elevators of the ribs, and the muscles of the diaphragm are all 

 brought into play. Contraction of these muscles elevates the 

 ribs and lowers the diaphragm, thus enlarging the cavity. The 

 air rushes in through the air passages and enters the lungs, 

 expanding the elastic lung tissue imtil it fills the enlarged 

 thoracic cavity. Immediately following inspiration the 

 diaphragm assumes its normal position, the intercostal and 

 elevator muscles relax, and thus the size of the pleural cavity 

 is reduced. The pressure of the walls of the thorax upon 

 the lungs forces the air out and expiration takes 

 place. 



The walls of the thorax move rhythmically normally sixteen 

 or eighteen times a minute. This rate is increased during 

 exercise. 



Composition of inspired and expired air. — The es- 

 sential components of air, from a physiological standpoint, 

 are oxygen, nitrogen and carbon dioxide. There are a few 

 other elements, but they have not been shown to have any 

 physiological significance. There are also accidental con- 

 stituents of the air varying with the locality. 



The following table, given by Howell, represents the 

 composition of ordinary inspired and expired air. 



Carbon 

 Nitrogen Oxygen dioxide Water 



Inspired 79 20.96 0.04 



Expired 79 16.02 4.38 .60 



