296 CONTAGIOUS PLEURO-PNEUMONIA 



susceptibility of the animals and the length of time during- 

 which exposure is continued. Sometimes not over 15, 20 or 

 30 per cent of the animals exposed will contract the disease 

 while at other times, 80 or go per cent may be infected. 

 The proportion of cases in which the disease proves fatal also 

 varies greatly — it may not exceed 10 per cent and it ma}' 

 reach 50 per cent. In general it ma}' be said that about 40 

 per cent of the exposed animals will contract the disease and 

 about one-half of these cases will prove fatal. 



§ 223. Morbid anatomy. Anatomically contagious 

 pleuro-pneumonia is characterized by a progressive interstitial 

 pneumonia with secondary hepatization of the lungs and exu- 

 dative pleuritis. Usually only one lung, the left as a rule, is 

 infected. The anatomical changes vary according to the 

 duration of the disease. 



The otherwise healthy lung .shows, in the initial stage, 

 small, circumscribed, inflammatory centers from the size of a 

 hazelnut to that of a walnut. The interlobular tissue in it 

 is hyperaemic, permeated by single hemorrhages and infiltrated 

 with serum. The reddened lobules of the lungs are surrounded 

 by bright margins, which are i to 2 mm. broad and which 

 are filled with a serous or lymphatic fluid. When the deposits 

 are superficial, the pleurae become opaque and covered with 

 slight clots. 



At the height of the disease there is a lobular pneumonia 

 with pleuritis which is usually spread over the greater part of 

 one lobe of the lung. The lung is considerably enlarged, of 

 firm consistence, very heavy (weighing up to one hundred 

 pounds), sinks in water and does not crackle when cut. 

 Its section appears marbled, in consequence of the interstitial 

 connective tissue having become thickened into broad lines 

 which vary in color from orange to dirty white and which 

 surround the darker colored lobules of the lung. The larger 

 lobules have a thickness of from .5 to 2 cm.; and the smaller 

 ones of from .25 to .50 cm. The color of the enclosed lobules 

 of the lungs depends on the duration of the process and varies 

 from brown-red to dirty yellow. The recently infected lobules 

 have a blood-red, reddish-brown or dark brown color (stage 



