386 THE HUMAN BODY. 



ity is diminished and air expelled from the lungs. When the 

 chest is enlarged the air the lungs contain immediately dis- 

 tends them so as to fill the larger space ; in so doing it be- 

 comes rarefied and less dense than the external air; and since 

 gases flow from points of greater to those of less pressure, 

 some outside air at once flows in by the air-passages and 

 enters the lungs. In expiration the reverse takes place. The 

 chest cavity, diminishing, presses on the lungs and makes the 

 air inside them denser than the external air, and so some 

 passes out until an equilibrium of pressure is restored. The 

 chest, in fact, acts very much like a bellows. When the bel- 

 lows are opened air enters in 

 consequence of the rarefaction 

 of that in the interior, which 

 is expanding to fill the larger 

 space; and when the bellows 



Fi. 126.-D5agramto illustrate the en- are cl Sed a g ain & is expelled. 



try of air to the lungs when the thoracic To make the bellows Quite 

 cavity enlarges. 



like the lungs we must, how- 

 ever, as in Fig. 126, have only one opening in them, that of 

 the nozzle, for both the entry and exit of the air; and this 

 opening should lead, not directly into the bellows cavity, but 

 into an elastic bag lying in it, and tied to the inner end of 

 the nozzle-pipe. This sac would represent the lungs and the 

 space between its outside and the inside of the bellows, the 

 pleural cavities. 



We have next to see how the expansion and contraction 

 of the chest cavity are brought about. 



The Structure of the Thorax. The thoracic cavity has 

 a conical form determined by the shape of its skeleton (Fig. 

 127), its narrower end being turned upwards. Dorsally, ven- 

 trally, and on the sides, it is supported by the rigid frame- 

 work afforded by the thoracic vertebrae, the breast-bone, and 

 the ribs. Between and over these lie muscles, and the 

 whole is covered in, air-tight, by the skin externally, and the 

 parietal layers of the pleurae inside. Above, its aperture is 

 closed by muscles and by various organs passing between the 

 thorax and the neck; and below it is bounded by the dia- 

 phragm, which forms a movable bottom to the, otherwise, 

 tolerably rigid box. In inspiration this box is increased in 

 all its diameters dorso-ventrally, laterally, and from above 

 down. 



