DISEASES OF THE RESPIRATORY SYSTEM. 161 



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 

 inflammation 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. 



The stage of effusion is characterized by the pouring out of 

 a semi-fibrinous liquid ; more or less filling the cavity and floating 

 in this fluid are flocculi of fibrine, blood, and epithelial cells. 



The exudation is called serous, fibrinous, or fibrino-serous. 



In the third stage this material may undergo fatty degenera- 

 tion and become absorbed, in which case resolution takes place; or, 

 on the other hand, it may become organized and form false mem- 

 branes. In the human subject we often find the lungs attached 

 to the walls of the thoracic cavity, in which case it becomes neces- 

 sary to cut these adhesions to remove the lungs; this is rarely 

 found in the equine, but is quite common in the dog. 



The serous part of the exudation may become absorbed by 

 the blood-vessels and the lymphatics; the character of the inflam- 

 mation we generally see in the horse is the subacute form, which 

 contains more serum than fibrine, and therefore so dangerous on 

 account of drowning out the lungs, as we say, and the animal dies 

 from exhaustion. 



Is this a unilateral or a bilateral disease? 



This is somewhat of a disputed point. 

 On post-mortem examination we find the fluid in both cavities. 

 This is claimed to be a peculiarity — that is, there is a connection 



