RESPIRATION 395 



short period of rest viz. : between the end of an expiration and the beginning 

 of an inspiration and a relatively long period of activity, including both 

 inspiration and expiration. The former may be regarded as the static, the 

 latter as the dynamic condition of the thorax. In the static condition, the 

 thorax and its contained and associated organs sustain a definite relation 

 one to another; in the dynamic conditions these relations undergo a change 

 the extent of which is proportional to the extent of the movements. 1 



THE STATIC CONDITION 



Relation of the Thoracic Organs. Intra-pulmonic Pressure : Intra- 

 thoracic Pressure. In the static condition of the thorax the lungs, by 

 virtue of their distensibility, completely fill all parts of the thorax not 

 occupied by the heart and great blood-vessels (Fig. 188). This condition is 

 maintained by the pressure of the air within the lungs, the mtra-pulmomc 

 pressure, which with the respiratory passages open, is that of the atmosphere, 

 760 mm. Hg. This relation persists so long as the thorax remains an 

 tight If the skin and muscles covering an intercostal space be removed the 

 lung can be seen in close contact with the parietal layer of the pleura gliding 

 by with each inspiration and expiration. If, however, an opening be now 

 made in the pleura sufficient to admit air, the lung immediately collapses and 

 a pleural cavity is established. A pleural cavity, therefore, does not exist 

 physiological conditions, it b POteptSri^nJ^ The pressure of air within 

 LfrWsuTSn^^ at the moment the air is admitted, 



the elastic tissue at once recoils and forces a large part of the air out of th 

 lung This is a proof that in the normal condition, the lungs, distended 1 

 atmospheric pressure from within, are in a state of elastic tension and ever 

 endeavoring to pull the pulmonic layer of the pleura away fror, , the^ pane^a 



lhe 



> It is a matter of discussion as to whe*er T "^ 



of the thoracic walls at the end of expiration A J^ for ^ purposes here intended 



men 



o e ora or 



if there is no absolute cessation, the ; movemen^ ^^ ^^ rec ogmzed that the forces both 

 a pause may be admitted. With this admiss , on* *, , h . R opposlte directog 



elastic and muscular, which are ^ways acting on^ the thora ^ ^ ^ are practlcally 



walk are stationary * 



