334 



RESPIRATION AND EXCRETION 



rubber stopper. Fasten two small toy balloons to the branches of the 

 tube. Close the small end of the jar with the stopper. Adjust the tube 

 so that the balloons shall hang free in the jar. If now the rubber sheet is 

 pulled down by means of the string, the air pressure in the jar is reduced 

 and the toy balloons within expand, owing to the air pressure down the 

 tube. When the rubber is allowed to go back to its former position, the 

 balloons collapse. *. 



Rate of Breathing and Amount of Air Breathed. During quiet 

 breathing, the rate of inspiration is from fifteen to eighteen times 



per minute ; this rate largely depends on 

 the amount of physical work performed. 

 About 30 cubic inches of air are taken in 

 and expelled during the ordinary quiet 

 respiration. The air so breathed is called 

 tidal air. In a "long" breath, we take 

 230 in about 100 cubic inches in addition to 

 cu. in. the tidal air. This is called complemental 

 air. By means of a forced expiration, it 

 is possible to expel from 75 to 100 cubic 

 inches more than tidal air; this air is 

 called reserve air. What remains in the 

 lungs, amounting to about 100 cubic 

 inches, is called the residual air. The 

 value of deep breathing is seen by a 

 glance at -the diagram. It is only by 

 this means that we clear the lungs of the 

 reserve air with its accompanying load of 

 carbon dioxide. 



Tidal Air 

 30 cu. in. 



Diagram showing the relative 

 amounts of tidal, comple- 

 mental, reserve, and resid- 

 ual air. The brace shows 



Respiration under Nervous Control. The 



muscular movements which cause an inspira- 



the average lung capacity tion are P ar % under tne control of the will, 

 for the adult man. but in part the movement is beyond our con- 



trol. The nerve centers which govern in- 

 spiration are part of the sympathetic nervous system. Anything of 

 an irritating nature in the trachea or larynx will cause a sudden expiration 

 or cough. When a boy runs, the quickened respiration is due to the fact 

 that oxygen is used up rapidly and a larger quantity of carbon dioxide is 



