APPARATUS OF RESPIRATION. 79 



The play of the ribs is rather more complex. These osseous 

 arches, extending from the vertebral column to the sternum 

 (with the exception of the lower ones), and articulated with 

 both, are much lower anteriorly than posteriorly, and thus 

 admit of elevation and depression. As the ribs are raised, 

 they rotate on themselves, and thus the cavity of the chest 

 becomes enlarged in all directions. They raise the sternum 

 with them. 



140. In expiration, the diaphragm is relaxed; the 

 external air acts on the walls of the chest ; the elasticity of 

 the lungs assists, and thus the air is expelled from the lungs ; 

 the intercostal muscles also cease to play. But in forcible 

 expiration generally, partly a voluntary act, the muscles of the 

 abdomen and others assist in forcing the air from the lungs. 



141. Under ordinary circumstances, the amount of air 

 taken in at each inspiration does not exceed the seventh part 

 of what they can contain ; on an average it may be estimated 

 at about the third of a quart. The number of the respiratory 

 movements varies with age and a variety of circumstances. 

 They are most frequent in the young; in the adult the 

 average is sixteen per minute. 



Thus, during ordinary circumstances, there enters into the 

 lungs of an adult male about 5^ quarts of air per minute, or 

 about 330 by the hour ; 7920 by the day. 



142. Sighing, yawning, hiccup, laughter, and sobbing, 

 are but modifications of the ordinary actions of respiration. 



Sighing is a deep and prolonged inspiration, not always 

 caused by a moral sentiment, but occasionally by a feeling 

 that the respiratory act does not proceed with sufficient 

 energy and rapidity. 



Yawning is an inspiration still deeper, accompanied with, 

 an almost involuntary and spasmodic contraction of the 

 muscles of the jaw and pendulous palate. 



Laughing seems to depend on a series of rapid movements 

 of the diaphragm ; sobbing differs but little from laughing, 

 though expressing passions and feelings of so opposite a cha- 

 racter. 



143. Mechanism of Respiration in other Animals. 

 The mechanism of respiration is essentially the same in all 

 mammals, birds, and reptiles ; in the two latter classes, how- 

 ever, the diaphragm is more or less completely absent, and in 

 consequence it is principally by the play of the ribs that the 

 air is drawn into the lungs ; in the chelonia (turtle and tor- 



