A STUDY OF RESPIRATION 219 



tube that represents the windpipe, and in this way the rub- 

 ber balloon is distended until it nearly tills the bell jar. At 

 the same time, the outside pressure of the air against the 

 sheet rubber forces it up into the bell jar, and its like- 

 ness to the form of the diaphragm becomes even more 

 apparent. 



Let us now seize the marble and pull the sheet rubber 

 downward. The cavity within the bell jar becomes larger, 

 and hence the inside pressure is less. More air rushes 

 down the glass windpipe and distends still further the rub- 

 ber lung. On releasing the marble we find that the dia- 

 phragm moves up again to its former position, and that some 

 of the air is forced out of the lung. 



The applications of the experiment to the action of the 

 lungs are evident, and we need call attention only to certain 

 points in which the experiment fails to illustrate the process 

 of respiration. In the first place, the glass bell jar allows 

 no movement forward and backward, and from side to side, 

 as do the walls of the chest. In the human body, there- 

 fore, a much greater expansion of the lungs is possible than 

 in this apparatus. Again, nothing corresponding to the rub- 

 ber tube is found in animals with lungs, for chest cavities 

 are always free from air. And, finally, the force that gives 

 to the human diaphragm its dome-shaped form is exerted 

 by the anterior pressure of the abdominal organs, not by 

 the pressure of the air, as is the case in our experi- 

 ment. 



Inspiration and Expiration. During inspiration, then, we 

 enlarge the chest cavity by pulling upward and outward 

 the front ends of the ribs, thus pushing ventrally the breast- 

 bone, and by pulling the diaphragm posteriorly. A greater 

 space is thus given for the lungs, and the air rushes in from 

 the outside, distending the elastic lungs and keeping them 

 in close contact with the walls of the chest cavity. Inspira- 

 tion requires a considerable amount of muscular effort, for 

 the cartilages attached to the ribs must be bent, and the 



