EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 649 



represented in Fig. 263, 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. It is to be remembered, of course, that the 

 interior surface of the lungs is multiplied greatly by the sub-? 

 division into alveoli. It is 

 estimated that the entire 

 inner surface of the lungs 

 amounts to as much as 90 

 square meters, over one hun- 

 dred times the skin surface 

 of the body. The atmos- 

 pheric pressure on the interior 

 surfaces of the lungs expands 

 these structures under normal 

 conditions until they fill the 

 entire thoracic cavity not 

 occupied by other organs. 

 However 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 

 occupied. If the wall of the 

 thorax is opened at any 

 point so as to make commu- 

 nication with the outside air, or, if the wall of the lung is pierced 

 so that the air can communicate with the pleural cavity from 

 the inside, then at once the lungs shrink in size, since the atmos- 

 pheric 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 

 constitutes an active expiration, which will drive some air out of the 

 lungs. It is evident, however, that after an active inspiration the 



Fig. 263. 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. 



