CHAPTER XI. 



THE PLEURA. 



DESCRIPTION. PLEURISY. 



PESSURE EFFECTS. 



The pleural cavity is a potential space between the 

 lung and the walls of the compartment that contains the 

 lung these walls are : The inner surface of the thorax, the 

 upper surface of the diaphragm and the outer surface of 

 the pericardium and the large blood vessels above the- 

 pericardium. The synovial membrane forming the pleu- 

 ral sac may be considered to have been "tucked" in, be- 

 tween the lung and these walls. The portion, of the pleura 

 that covers the lung is termed the visceral layer, while 

 that lining the walls of the receiving cavity is the parietal 

 layer. The external surface of both the visceral and the 

 parietal layers is rough and fibrous and is attached to the 

 structures it covers, especially over the lung and dia- 

 phragm, whereas, the free surface is smooth and glistening 

 and secretes a fluid which permits the lung to glide readi- 

 ly over the contiguous surfaces. The usual quantity of 

 the secretion is about two drachms, but it may be enor- 

 mously increased when the pleura is diseased, as, for in- 

 stance, in pleurisy with effusion. We have stated that the 

 cavity of the pleura is potential, i.e., that it does not exist 

 as an actual or real cavity, except in pathological condi- 

 tions, such as pleurisy, pneumo-thorax, etc., the reason be- 

 ing, that the two layers, visceral and parietal, are in close 

 contact, excepting along the lower border, where the parie- 

 tal layer extends for about one inch anteriorly, and two 

 and a half inches posteriorly, below the visceral layer, so 

 as to accommodate the descent of the lung in inspiration. 



