RESPIRATION 191 



the pleural bloodvessels are passively congested. As 

 fluid is poured out, the contractility of the lung is 

 gradually able to assert itself, and the organ continues 

 to recede pari passu with the encroachment of the fluid. 

 As it does so the negative pressure gradually diminishes, 

 but it requires a large effusion to produce a really positive 

 pleural pressure. Hence the necessity for aspiration in 

 removing small effusions. If one lung collapses, the 

 other, being no longer pulled on from its mediastinal 

 side, contracts up to a considerable extent, and the 

 healthier it is i.e., the greater its 'tone' the more 

 does it do so. Hence if a healthy man becomes suddenly 

 the subject of a pneumothorax, he loses not only the 

 lung of the affected side, but a large part of the other as 

 well, and his dyspnoea is proportionately great. 



During forcible expiration the intra thoracic pressure 

 becomes positive, with the result that the two layers of 

 the pleura are squeezed tightly together, and even 

 although the lung be penetrated by a punctured wound, 

 pneumothorax may not result. This point has been 

 graphically put by Sir James Barr :* ' I do not know 

 if any of you have encountered a man who was making 

 indiscriminate use of a knife. I have, and for a short 

 time I got the worst of the conflict. If you should meet 

 such an individual, you can deal with him as you think 

 best, but if you find him making a lunge for your chest, 

 I would strongly advise you to let your breath out before 

 he does it for you. In this way the knife or dagger has 

 another inch to travel before it reaches your chest, and 

 you establish a positive pressure within your thorax 

 * The Bradshaw Lecture, 1907. 



