218 



STUDIES IN PHYSIOLOGY 



are made to contract, the central tendon is pulled posteriorly 

 upon the stomach, liver, and other abdominal organs, and 

 these in turn force outward the wall of the abdomen. By 

 the action just described the size of the chest cavity is 

 increased in its third dimension, 

 namely, from its anterior to its pos- 

 terior end. 



How the Lungs are filled with Air. 

 In order to understand the way the 

 lungs are inflated, we may study to 

 good advantage the action of the ap- 

 paratus represented in Fig. 106. 1 

 Over the bottom of a bell jar is 

 stretched a piece of sheet rubber, in 

 the center of which a marble is tied. 

 A toy balloon or the lungs of a cat 

 are fastened to the lower end of one 

 of the glass tubes passing through 

 the rubber cork in the top of the bell 

 jar. To the upper end of the second 

 glass tube is attached a piece of rub- 

 ber tubing which can be tightly closed 

 by a clamp. The bell 'jar is designed 

 to represent the walls of the chest cavity, the sheet of rubber 

 answers for a diaphragm, while the glass tube and rubber 

 balloon function for the windpipe and one lung. 



We learned in the first chapter that the air exerts a pres- 

 sure of fifteen pounds on every square inch of surface. In 

 the apparatus we are describing, this pressure is the same 

 on the inside and outside of the bell jar and of the balloon, 

 and above and below the sheet rubber. But if we exhaust 

 as much air as possible from the bell jar through the rubber 

 tube, and then fasten the clamp, we reduce the pressure 

 on the inside of the bell jar, and therefore outside the bal- 

 loon. Air is then forced by atmospheric pressure down the 

 1 See - ; Laboratory Exercises," No. 38. 



FIG. 106. Apparatus to 

 illustrate the Inflation 

 of the Lungs. 



