392 THE HUMAN BODY 



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 pres- 

 sure is restored. The chest, in fact, 

 acts very much like a bellows. 

 When the bellows are opened air 

 FIG lie Diagram to illustrate enters in consequence of the rare- 



^ 11 ^ of that in the interior > 

 which is expanding to fill the larger 



space; and when the bellows are closed again it is expelled. To 

 make the bellows quite like the lungs we must, however, as in 

 Fig. 116, 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 ly- 

 ing 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. 117), its nar- 

 rower end being turned upwards. Dorsally, ventrally, and on the 

 sides, it is supported by the rigid framework 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 diaphragm, which forms a movable bottom to the, other- 

 wise, tolerably rigid box. In inspiration this box is increased in 

 all its diameters dorsiventrally, laterally, and from above down. 



The Vertical Enlargement of the Thorax. This is brought 

 about by the contraction of the diaphragm which (Figs. 1 and 118) 

 is a thin muscular sheet, with a fibrous membrane, serving as a 

 tendon, in its center. In rest, the diaphragm is dome-shaped, 

 with its concavity towards the abdomen, being supported in that 

 position by the pressure of the underlying abdominal organs. 

 From the tendon on the crown of the dome striped muscular fibers 



