RESPIRATION. 343 



That this exchange of gases may continue, it is of primary impor- 

 tance that the air within the alveoli be removed as rapidly as it is 

 vitiated. This is accomplished by an alternate increase and decrease 

 in the capacity of the thorax, accompanied by corresponding changes 

 in the capacity of the lungs. During the former there is an inflow 

 of atmospheric air (inspiration), during the latter an outflow of intra- 

 pulmonary air (expiration). The continuous recurrence of these 

 two movements brings about that degree of pulmonary ventilation 

 necessary to the normal exchange of gases between the blood and the 

 air. The two movements together constitute a respiratory act or 

 cycle. 



In the course of the respiratory cycles the thorax presents alter- 

 nately a 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 ancTiFs 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.* 



Static Relations. Intra-pulmonary Pressure. In the static 

 condition of the thorax the lungs, by virtue of their distensibility, 

 completely fill all parts of the thoracic cavity not occupied by the 

 heart and great blood-vessels. This condition is maintained % the 

 pressure of the air within the lungs, the intr a- pulmonary pressure, which 

 with the respiratory passages open, is that of the atmosphere, 760 

 mm. Hg. This relation persists so long as the thoracic cavity remains 

 air-tight. If the skin and muscles covering an intercostal space be re- 

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

 the pleura gliding by with each inspiration and expiration. If, how- 

 ever, an opening be now made in the pleura sufficient to admit air, the 

 lung immediately collapses and a pleural cavity is established (pneu- 

 mothorax). The pressure of air within and without the lung counter- 

 balancing, at the moment the opening is made, the elastic .tissue at 

 once recoils and forces a large part of the air out of the lung. This 

 is a proof that in the normal condition, the lungs, distended by at- 

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

 ever endeavoring to pull the visceral layer of the pleura away from 



* It is a matter of dispute as to whether or not there is an absolute cessation of 

 movement of the thoracic walls at the end of expiration. A graphic record of the 

 movement shows that if there is no absolute cessation, the movement is so slight that, 

 for the purposes here intended, a pause may be admitted. With this admission it 

 is, however, recognized that the forces, both elastic and muscular, which are always 

 acting on the thoracic walls, though in opposite directions, have not ceased to act, 

 but have become so nearly equal that for a brief period they are practically in a con- 

 dition of equilibrium, during which the thoracic walls are stationary. 



