144 PHYSIOLOGY AND HEALTH. 



CHAPTER VIII. 



Lungs must have Capacity to receive sufficient Air. Action of 

 Respiration performed by the Muscles of Chest and Diaphragm. 

 Ribs spread outward in Inspiration. Action of Diaphragm 

 presses the Abdomen downward and outward. 



323. IT is not only necessary that the lungs should be in 

 good health, and be supplied with pure air, but they should 

 be able to receive it in sufficient quantity. This implies that 

 the chest should be of the natural size, and that it should 

 have the due power and opportunity of expansion and con- 

 traction. 



324. Although it is absolutely necessary that the air 

 reach the blood in the lungs, yet it has no active power to 

 get there. It is merely passive. It does not enter of its own 

 accord, but it is pressed into these organs, when, by the en- 

 largement of the cavity, a vacuum, or rather, more room, is 

 made for it. Nor have the lungs any active power of expan- 

 sion. They, too, are merely passive. Their air-cells do not 

 extend themselves, and thus press the walls of the chest 

 outward. But, when these walls are extended, the air rushes, 

 or rather it is pressed, into these air tubes and cells, and com- 

 pels the lungs, thus filled with air, to swell and completely fill 

 the cavity of the chest. 



325. The structure of the chest is arranged like the com- 

 mon bellows for expansion and contraction. ( 270 277, pp. 

 122, 125.) The bony framework is furnished with joints, on 

 which the ribs move. The muscular covering contracts and 

 sets this framework in motion, while the diaphragm draws 

 down, and both cooperate to enlarge the internal capacity 

 of the pulmonary cavity; then the ribs fall, and the abdom- 

 inal muscles press the diaphragm up, and both combine to 

 diminish this cavity. 



326. In Fig. XXL, the full black lines represent the out- 

 line of the chest and the abdomen when the lungs are empty, 

 and the dotted lines represent the same when the lungs are 



