136 ANATOMY FOR NURSES. [CHAP. XII. 



and the air rushes out of the trachea until equilibrium be once 

 more established. This constitutes an expiration. An inspira- 

 tion and an expiration make a respiration. 



As in the heart, the auricular systole, the ventricular systole, 

 and then a pause, follow in regular order, so in the lungs, 

 the inspiration, the expiration, and then a pause succeed one 

 another. Each respiratory act in the adult is ordinarily repeated 

 from fifteen to eighteen times per minute. But this rate varies 

 under different circumstances, one of the most important of 

 which is age. The average rate in the newly born infant has 

 been found to be forty-four per minute, and at the age of five 

 years, twenty-six per minute. It is reduced between the ages 

 of fifteen and twenty to the normal standard. 



A condition of rest or activity readily influences the number 

 of respirations per minute. They are always less frequent 

 during sleep, and are markedly increased by severe muscular 

 exercise. 



Respiration is an involuntary act. It is possible for a short 

 time to increase or retard the rate of respiration within certain 

 limits by voluntary effort, but this cannot be done continuously. 

 If we intentionally arrest the breathing or diminish its fre- 

 quency after a short time the nervous impulse becomes too 

 strong to be controlled, and the movements will recommence 

 as usual. If, on the other hand, we purposely accelerate res- 

 piration to any great degree, the exertion soon becomes too 

 fatiguing for continuance, and the movements return to their 

 normal standard. 



The nervous impulses which cause the contractions of the 

 respiratory muscles arise in the medulla oblongata, travel down 

 the spinal cord, and out along the phrenic and intercostal 

 nerves. If the portion of the medulla oblongata, where these 

 nervous impulses arise, be removed or injured, respiration 

 ceases, and death at once ensues. This part of the medulla is 

 known as the respiratory centre, and is also often called the 

 "vital knot." 



The effects of respiration upon the air within the lungs. At 

 birth the lungs contain no air. The walls of the air-sacs are in 

 close contact, and the walls of the smaller bronchial tubes or 

 bronchioles collapsed and touching one another. The trachea 

 and larger bronchial tubes are open, but contain fluid and not 



