Excretion. 63 



two layers, one covering the lungs, the other lining the 

 inside of the ribs; between the two layers is a small space. 

 The two layers of the pleura, in health, glide over one 

 another without friction, being oiled by the secretions, as 

 a result of inflammation, the layers become dry and pain 

 is the result; later on an excessive discharge of fluid may 

 take place, constituting water in the chest (hydrothorax). 



