148 PRACTICE OF EQUINE MEDICINE. 



It may accompany blood diseases, as glanders, or it is often 

 epizootic where influenza is prevailing in some of its forms. 



It may also be caused by foreign bodies in the pleural sac, 

 blood, pus, or air in the sac, or, again, necrosis of the ribs or ver- 

 tebrae. 



Inflammation of these structures, with a secondary pleurisy 

 in some cases. 



Changes in the atmosphere often aid to cause pleurisy. It is 

 more generally a secondary disease; the animal may be suffering 

 with a laryngitis, and you think he will get well; but the fever 

 keeps high, and after a time subacute pleurisy is developed. 



Nowadays it is said to be caused by micro-organisms. 



What is the pathological anatomy of pleurisy ? 



The course of an inflammation of a serous membrane is — 

 hyperemia, followed by exudation of lymph, the effusion of fluid, 

 its absorption, and the adliesion of the membrane. For study, 

 pleurisy may be divided into three stages. 



The first stage, or dry stage, or stage of engorgement or conges- 

 tion. There is an irregular redness of the membrane, with little 

 specks of exudation, called coagulable lymph; this can be produced 

 by an injection of oxalic acid. At first the membrane is dry, red- 

 dened, with an increased network of engorged vessel; it loses its 

 shining appearance and becomes opaque, from the shedding of the 

 epithelium and the commencing exudation. 



The second stage is characterized by the copious exudation of 

 lymph (more or less covering the membrane, giving it a dull, cloudy, 

 or shaggy appearance) from the clumps of fibrine; if the inflamma- 

 tion ceases at this point, it is called dry pleurisy. The amount of 

 exudation depends on the amount of inflammation. When the 

 inflammatory process is acute, the exudation consists most entirely 

 of fibrine factors, which coagulate; often these layers of fibrine 

 can be seen covering the sides of the pleural cavity, both on the 

 visceral and the parietal layers. 



The fibrine is a thick, yellowish material, deposited in layers, 

 patches, or shreds, according to the amount, causing the membrane 

 to become very much thickened; it is infiltrated with serum and 

 some leucocytes, it rarely being puriform. In the milder forms of 

 inflammation of the pleura, serum is poured out. The albuminous 

 part of the blood and serum is transuded and has a tendency to 

 gravitate to the bottom, and often fills up the sac. 



