THE RESPIRATORY ORGANS 189 



the number of deaths has been steadily decreasing. In Massa- 

 chusetts, for example, while in 1870 the number of deaths was 

 thirty-six in ten thousand, there are about eighteen in ten 

 thousand at the present time. Tuberculosis is largely a pre- 

 ventable disease, and if each person will do what he can to 

 pass along information as to its cause and the methods of 

 avoiding it, he will help to reduce the number of cases. 



Pleurisy. The ease of motion which we have noticed in 

 the lungs is dependent upon the free moving of the layers of 

 the pleura upon each other, and this in turn is dependent up- 

 on the presence of a watery liquid secreted by their glandular 

 surfaces. It sometimes happens that the pleura ceases to 

 perform its proper function, becomes inflamed, or adheres to 

 adjacent tissues. In all such cases the movements of the 

 lungs become difficult and, as a result, breathing is not so 

 free and sometimes becomes distinctly painful. The trouble 

 is called pleurisy, and may result from various causes, of 

 which a common cold is one. While painful, this trouble is 

 rarely dangerous, and usually passes off with proper treat- 

 ment at the hands of a physician. 





