EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 571 



glands. All these organs, therefore, lie outside the lungs. A 

 schematic view of these relations is represented in Fig. 234. 



The Thorax as a Closed Cavity. The thorax is a cavity entirely 

 shut off from the outside and from the abdominal cavity. In this 

 cavity lie the lungs and the various viscera enumerated above. 

 The lungs may be considered as two large, membranous sacs, as 

 represented in Fig. 234, the interior of which communicates freely 

 with the outside air through the trachea, glottis, etc., while the 

 outside of the sacs is protected from atmospheric pressure by the 

 walls of the chest. The atmospheric pressure on the interior 

 surfaces of the lungs expands these structures under normal con- 

 ditions until they fill the en- 

 tire thoracic cavity not occu- 

 pied by other organs. How- 

 ever the size of the chest 

 cavity varies, that of the 

 lungs must change accord- 

 ingly; so that at all times the 

 lungs fully fill up every part of 

 the cavity not otherwise occu- 

 pied. If the wall of the thorax 

 is opened at any point so as to 

 make communication with the 

 outside air, or, if the wall of 

 the lung is pierced so that the 

 air can communicate with the 

 pleural cavity, then at once 

 the lungs shrink in size, since 

 the atmospheric pressure is 

 then equalized on the outside 

 and the inside of the sacs. 

 We may consider, therefore, 

 that the thoracic cavity is 



much larger than the lungs, and that the latter are blown out to 

 fill this cavity by the atmospheric pressure on the inside. 



The Normal Position of the Thorax Inspiration and Expira- 

 tion. During life the size of the thorax is continually changing with 

 the respiratory movements. But the size and position taken at the 

 end of a normal expiration may be regarded as the normal position 

 of the thorax ; that is, its position when all of the muscles of respira- 

 tion are at rest, and substantially, therefore, the position of the 

 thorax in the cadaver. Starting from this position, any enlarge- 

 ment of the thorax constitutes an active inspiration, the result of 

 which will be to draw more air into the lungs ; while starting from 

 the normal position any diminution in the size of the thorax 



Fig. 234. Schema to indicate the re- 

 lations of the parietal and visceral layers of 

 the pleural sacs, and the position of the me- 

 diastinal space: P, the potential pleural 

 cavity in each sac; M, the mediastinal 

 space; R.L. and L.L., the cavity of the 

 right and the left lung, respectively; T, the 

 trachea. The outlines of the pleura on each 

 side are represented in dotted lines. 



