MOVEMENTS OF THE THORAX. 35 



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 bellows are- 

 opened air enters in con- 

 sequence of the rarefaction 

 of that in the interior, 

 which is expanding to fill 

 the larger space; and when 

 the bellows are closed 



... 11 j m FlG - 108. Diagram to illustrate the en 



again It IS expelled. 10 try of air to the lungs when the thoracie 



make the bellows quite 



like the lungs we must, however, as in Fig 108, 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 pleura! cavi- 

 ties. 



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. 

 109), its narrower end being turned upwards. Dorsally, 

 ventrally, and on the sides, it is supported by the rigid 

 framework afforded by the dorsal 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 in- 

 creased in all its diameters dorso-ventrally, 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 110) is a thin muscular sheet, with a fibrous 

 membrane, serving as a tendon, in its centre. In rest, the* 



