ORGANS OF BREATHING. 



349 



stcreteb a slimy fluid (mucus), which is known as " phlegm," when it is 

 discharged from the vrindpipe. 



The liiiiijs are composed of a spongy substance, which is made up of a 

 vast number of small lobules that are connected together and kept separate 

 by cellular tissue. Each of these lobules is composed of many minute ail' 

 cells, and is supplied with a small bronchial tube, which conveys air to 

 these cells. The capillary blood-vessels are distributed through the walls of 

 the air-cells, so that while the air in one cell acts on one side of a capillary, 

 the air ill the adjoining cells acts on the other side. The blood thus brought 

 into extremely close proximity with the air contained in the cells, while 

 traversing their walls, takes up the necessary supjily of oxygen for the 



Kuhie^y.^ 



P.crlitnu 



\\ Bladder 



„ „ i 



JliL.iuj; Urethra 



Interna! organs of the he 



requirements of the system, and, on its return to the lungs, gives off the 

 carbonic acid which is absorbed from the various tissues. The skin, to a 

 , small extent, also secretes carbonic acid. 



The pleiiros are two smooth and glistening membranes which line the 

 cavity of the chest and cover the lungs, thus forming two closed sacs ; their 

 office being to prevent friction between the lungs and the walls of the chest. 

 They secrete serous fluid (p. 13) with which to lubricate their inner surface. 



General Remarks on Pneumonia, Pleurisy, and 

 Bronchitis. 



The lungs are so intimatel}' connected with the bionclii.il tubes 

 and the pleurae, that they are rarely affected without either of the 

 other two structures being also implicated ; and, if either of tliem 



