198 APPLIED PHYSIOLOGY 



while all the air is not changed with each inspiration, yet 

 there is a free mingling of the fresh with the impure air. 

 The cilia also intercept particles of dust which the nose 

 and pharynx have failed to remove. 



344. Rate of breathing. In health an inspiration occurs 

 with every four heart beats, or about eighteen times each 

 minute, but in exercise its rate may be increased to sixty 

 or seventy times a minute. A baby breathes about forty 

 times a minute. The rate slowly diminishes until, at 

 eighteen years of age, it is the same as in a man. 



An inspiration takes about five sixths as long as expira- 

 tion, but the regularity and force of both inspiration and 

 expiration can be varied indefinitely. Respiration usually 

 goes on without a person's knowledge or thought, yet it is 

 somewhat under the control of the will in talking, blow- 

 ing, and other actions. 



345. Modifications Of breathing. Coughing is a forcible expi- 

 ration in which the closed vocal cords are suddenly blown open with 

 force. 



Sneezing \s> a sudden expiration in which air is driven mainly through 

 the nose. 



Blowing is a long forcible expiration in which air is forced in a steady 

 stream through a small opening in the lips. 



Laughing and crying are each a succession of short expirations. 

 They sound so much alike that it is often impossible to tell which a 

 child is doing. 



Sobbing is a succession of short inspirations. 



Hiccoughing is a single inspiration caused by a sudden contraction of 

 the diaphragm. 



Snoring is a sound produced during inspiration by air passing over 

 the soft palate. It is usually due to air passing through both the nose 

 and the mouth at the same time. 



Gaping or yawning is a long and deep inspiration and expiration 

 through the open mouth, while the muscles of the throat are strongly 

 contracted. 



