456 



RESPIRATION 



The chest is never at rest, but is either in the position of inspiration or 

 expiration, and hence, the respiratory cycle is one of constant activity. 

 Between the end of every expiration and the beginning of the succeed- 

 ing inspiration the thorax exhibits a condition of quiescence which is 

 sometimes erroneously designated as the respiratory pause. Thus, 

 it may be stated that the respiratory cycle includes a dynamic and a 

 static phase, the former consisting of the inspiratory and expiratory 

 periods, and the latter of this period of comparative passiveness and 

 rest. It should be remembered, however, that the elasticity of the 

 lungs and of the parts forming the thoracic walls does not cease to 

 act even during the respiratory interims, when the muscle tissue is 

 actually in a state of rest. For this reason, it may justly be said that 

 a true pause does not exist during life, although it may be produced 

 experimentally. Hence, the term static should be used solely in a 

 relative sense. 



FIG. 234. DIAGRAM ILLUSTBATING THE COURSE OF THE PLEURA. 

 T, trachea; L, lung; H, heart; A, abdominal cavity; C, collapsed lung (the rest of 

 this cavity being filled with air (pneumothorax) ; V, visceral pleura; P, parietal pleura 

 reflected from root of lung (dotted line). 



A. THE STATIC PHASE. 



At the end of expiration the thorax and the organs contained 

 therein maintain for a very brief time a position from which several 

 of the fundamental principles of respiration may readily be inferred. 

 The entire cavity, with the exception of the mediastinal space, is 

 filled by the lungs which are everywhere in immediate contact with 

 the internal surface of the chest wall. Externally, they are protected 

 against the atmospheric pressure by the relatively solid framework 

 of the thorax and atmospheric pressure prevails in all the intrapul- 

 monary spaces and passages. 



