370 DISEASES OP CATTLE. 



contract the disease when a large herd is exposed ; but, on the other 

 hand, 80 or 90 per cent may be affected. 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 may be said 

 that about 40 per cent of the exposed animals will contract the dis- 

 ease and about one-half of these cases will prove fatal. 



Post-mortem appearances. — Owing to the complexity of the struc- 

 ture of the lung tissue, its ramifications of bronchial tubes and blood 

 vessels and its abundant supply of lymphatics, the pathological 

 changes in pleuro-pneumonia are interpreted with great difficulty. 

 Furthermore there are certain kinds of pneumonia which present 

 some resemblances to- pleuro-pneumonia and which may therefore be 

 confused with it in some of its phases. 



If we kill an animal affected with acute pleuro-pneumonia and 

 examine the cavity of the chest and lungs, the following appearances 

 will be noted : 



The thorax may contain more or less serum, which may be clear or 

 clouded. There may be firm adhesions of different parts of the lungs 

 to the chest wall, the extent of which depends on the stage and 

 severity of the disease. The diseased lobes are unusually large and 

 exceedingly firm to the touch. The weight of a single large lobe may 

 reach 40 pounds. Usually only one side is affected, often but a single 

 lobe, and this most commonly the large or principal lobe. The pleura 

 may be covered with one or more layers of a firm, elastic, grayish 

 membrane, which varies in thickness and which sometimes may be 

 pulled away entirely. Sometimes it is absent. The pleura, however, 

 is opaque and apparently very much thickened. This is due to the 

 diseased condition of the connective tissue beneath the pleura, as 

 will be explained later. When an affected lobe is cut through at 

 right angles to its long diameter, the cut surface will present a variety 

 of interesting changes. In the first place the spaces between the 

 small subdivisions of the lung (the lobules), which in the healthy 

 lung are barely visible, are distended with a yellowish white, usually 

 quite firm, substance, which is coagulated fibrin. The cut surface 

 thus appears divided up into small fields by yellowish white bands of 

 varying thickness running in various directions through the lung tis- 

 sue and beneath the pleura. (PI. XXXII.) These bands may appear' 

 honeycombed and the spaces filled with yellowish fluid (serum) or they 

 may be uniformly solid. It will also be noticed that the space immedi- 

 ately outside of and around the artery, vein, and air tube is similarly 

 broadened by fibrinous deposits. Some authorities look upon these 

 bands as constituting the so-called " marbling" of pleuro-pneumonia. 



In addition to these changes which have taken place in the connec- 

 tive tissue between the lobules, the lung tissue itself may be mark- 

 edly altered. Certain areas of the cut surface may be very firm in 

 texture and brownish red in color. The cut surface is granular or 



