DISEASES OF THE RESPIRATORY ORGANS. 128 
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 fluid 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. 
Symptoms.—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 
