MORBID ANATOMY 447 



Seriously aflfected animals remain standing if they have 

 sufficient strength, but those which lie down are said to always 

 lie on the affected side. 



The proportion of animals that become affected after being 

 exposed varies according to the virulence of the outbreak, the 

 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 90 per cent ma}^ be infected. The 

 proportion of cases in which the disease proves fatal also varies 

 greatly, it may not exceed 10 per cent and it may reach 50 

 per cent. In general it maj' 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. 



§ 355- 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 

 affected. The anatomical changes vary according to the dura- 

 tion of the disease. 



The otherwise healthy lung shows, in the initial stage, 

 small, circum-scribed, inflammatory centers from the size of a 

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

 is hj'peremic, 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 plurce 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 consistency, very heavj'^ (weighing up to one hundred 

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

 section appears marbled, in consequence of the interstitial con- 

 nective tissue having become thickened into broad lines which 



