66 ELEMENTARY PH YSIOLOGY. 



watery fluid which keeps the walls of the chest and 

 the surface of the lungs moist, and thus prevents 

 friction which would otherwise be produced in the 

 movement of breathing. The pleura is shown at (7 

 on section of the ribs). An inflammation of this 

 membrane is called pleurisy. When both pleura and 

 lungs are inflamed it is called pleura-pneumonia. 



Health of the -^ rom w ^ a * we Bave learned of the 

 Respiratory structure of the chest and the action of 

 the lungs in breathing, we can not fail 

 to see that the healthy and natural action of the respi- 

 ratory system requires perfect freedom of motion or 

 expansion of every part concerned in the vital act of 

 respiration. It is Nature's plan that every air cell 

 in the lungs should perform its appointed part with 

 all the rest, at every breath. If the habit of breath- 

 ing, full and deep, while the chest is perfectly free 

 to expand and the body is in erect or straight posi- 

 tion, is well formed, every lung-cell will be filled at 

 each inspiration. But if the chest is in any degree 

 restricted and compressed, the lungs will be but 

 partly filled, and many of the air-cells will lose their 

 elasticity, and finally become utterly useless. Such 

 an injurious interference with the natural expansion 

 of the chest may be produced by habitual unnatural 

 positions in sitting or walking, or by wearing the 

 clothing too tight about the body. 



We have learned that the flow of blood 

 through the vessels of the circulatory 



