384 THE HI MAN BODY. 



c_/^ 



is called the pleura ; that point at which the pleura is 

 wanting is called the root of the lung and is on its 

 median side; it is there that its bronchus, blood-vessels and 

 nerves enter it. At the root of the lung the pleura turns 

 back and lines the inside of the chest cavity, as represented 

 by the dotted line in the diagram Fig. 3. The part of the 

 pleura attached to each lung is its visceral, and that attached 

 to the chest-wall its parietal layer. Each pleura thus forms 

 a closed sac surrounding a pleural cavity, in which, during 

 health, there are found a few drops of lymph, keeping its 

 surfaces moist. This lessens friction between the two layers 

 during the movements of the chest-walls and the lungs; for 

 although, to insure distinctness, the visceral and parietal 

 layers of the pleura are represented in the diagram as not in 

 contact, that is not the natural condition of things; the lungs 

 are in life distended so that the visceral pleura rubs against 

 the parietal, and the pleural cavity is practically obliterated. 

 This is due to the pressure of the atmosphere exerted through 

 the air-passages on the interior of the lungs. The lungs are 

 extremely elastic and distensible, and when the chest cavity 

 is perforated each shrivels up just as an indian-rubber blad- 

 der does when its neck is opened; the reason being that then 

 the air presses on the outside of each with as much force as 

 it does on the inside. These two pressures neutralizing one 

 another, there is nothing to overcome the tendency of the 

 lungs to collapse. So long as the chest-walls are whole, how- 

 ever, the lungs remain distended. The pleural sac is air-tight 

 and contains no air, and the pressure of the air around the 

 Body is borne by the rigid walls of the chest and prevented 

 from reaching the lungs; consequently no atmospheric pres- 

 sure is exerted on their outside. On their interior, however, 

 the atmosphere presses with its full weight, 

 equal to about 90 centigrams on a square 

 centimeter (14.5 Ibs. on the square inch), and 

 this is far more than sufficient to dis- 

 tend the lungs so as to make them com- 

 pletely fill all the parts of the thoracic cav- 

 ity not occupied by other organs. Suppose 



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me mngs'Khl^ho- b J a cork through which two tubes pass, 

 rax. one of which, , leads into an elastic bag, 



d, and the other, c, provided with a stop-cock, opens freely 



