DISEASES OF THE EESPIRATOEY ORGANS. 123 



practical purposes. The smooth, shiny surface of the lung, as well as 

 the smooth, shiny surface so familiar on the rib, is the plura. In 

 health this surface is always moist. A jfiuid is thrown off by the 

 pleura, which causes the surface to be constantly moist. This is to 

 prevent the effects of friction between the lungs and the walls of the 

 chest and other contiguous parts which come in contact. It must be 

 remembered that the lungs are dilating each time a breath is taken 

 in, and contracting each time a breath of air is expelled. It may be 

 readily seen that if it were not for the moistened state of the surface 

 of the pleura the continual dilatation and contraction and the conse- 

 quent rubbing of the parts against each other would cause serious 

 friction. 



Inflammation of this membrane is called pleurisy. Being so closely 

 united with the lung, it can not always escape participation in the 

 disease when the latter is inflamed. Pleurisy may be due to the 

 same predisposing and exciting causes as mentioned in the beginning 

 of this work as general causes for diseases of the organs of respi- 

 ration, such as exposure to sudden changes of temperature, confine- 

 ment in damp stables, etc. It may be caused also by wounds that 

 penetrate the chest, for it must be remembered that such wounds 

 must necessarily pierce the pleura. A fractured rib may involve 

 the pleura. The inflammation following such wounds may be cir- 

 cumscribed ; that is, confined to a small area surrounding the wound, 

 or it may spread from the wound and involve a large portion of the 

 pleura. The pleura may be involved secondarily when the heart or 

 its membrane is the primary seat of the disease. It may occur in 

 conjunction with bronchitis, influenza, and other diseases. Diseased 

 growths that interfere with the pleura may induce pleurisy. The 

 most frequent cause of pleurisy is an extension of inflammation from 

 adjacent diseased lung. It is a common complication of pneu- 

 monia. Pleurisy will be described here as an independent affection, 

 although it should be remembered that it is very often associated 

 with the foregoing diseases. 



The first lesion of pleurisy is overfilling of the blood vessels that 

 ramify in this membrane and dryness of the surface. This is fol- 

 lowed by the formation of a coating of coagulated fibrin on the 

 diseased pleura and the transudation of serum which collects in the 

 chest. This serum may contain flakes of fibrin and it may be straw 

 colored or red from an admixture of blood. The quantity of this 

 accumulation may amount to several gallons. 



Sym/ptoTtis. — ^When the disease exists as an independent affection 

 it is ushered in by a chill, but this is usually overlooked. About the 

 first thing noticed is the disinclination of the animal to move or turn 

 around. When made to do so he grunts or groans with pain. He 

 stands stiff ; the ribs are fixed — that is, they move very little in the 



