OF EXHALATION AND THE SECRETIONS. 75 



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 tortues (turtle and tor- 

 toise), and the batrachia (frogs, salamanders, &c.), the thorax 

 is not formed so as to act as a suction pump, and accord- 

 ingly these animals swallow the air by a sort of deglutition. 



OF EXHALATION AND THE SECRETIONS. 



144. We have seen how the nutrient matter is dis- 

 tributed to all parts of the body by means of the blood ; we 

 have now to examine how the matters contained in the 

 general mass escape from it, whether into the interior or 

 directly from the body. 



145. Nutrition we have seen to be effected in two 



