RESPIRATION 515 



the bronchioles, are without this support, and are surrounded 

 by a specially well-developed circular band of non-striped 

 muscle — the bronchial muscle — which governs the admission 

 of air to the infundibula and air sacs. 



II PHYSIOLOGY. 



I. Physical Considerations. 



The lungs are packed in the thorax round the heart, 

 completely filling the cavity. 



They may be regarded as two compound elastic-iualled, 

 sacs, which completely fill an air-tight box w^ith movable 

 walls — the thorax — and which communicate with the exterior 

 by the windpipe or trachea. 



No space exists between the lungs and the sides and base 

 of the thorax, so that the so-called pleural cavity is simply a 

 potential space. 



The lungs are kept in the distended condition in the 

 thoracic cavity by the atmospheric pressure within them. 



Their elasticity varies according to whether they are 

 stretched or not. As they collapse, their elastic force 

 naturally become less and less, as they are expanded, greater 

 and greater. Taken in the average condition of expansion 

 in which they exist in the chest, the elasticity of the excised 

 lungs of a man is capable of supporting a column of mercury 

 of about 30 mm. in height, so that they are constantly 

 tending to collapse with this force. 



But the inside of the lungs freely communicates with the 

 atmosphere, and this, at the sea-level, has a pressure of 

 about 760 mm. Hg. During one part of respiration, this 

 pressure becomes a few mm. less, during another part a few 

 mm. more; but the mean pressure of 760 mm. of mercury 

 is constantly expanding the lung, and acting against a 

 pressure of only 30 mm. of mercury, tending to collapse the 

 lung (fig. 208> 



Obviously, therefore, the lungs must be kept expanded 

 and in contact with the chest wall. 



When a pleural cavity is opened, the distribution of 



