22 DEPAKTMENT OF AGEICULTUKE. 



is so large, that from a normal weight of four or five pounds, a lung attains to ten, twenty, 

 forty, and I have seen one as high as fifty-four pounds in weight. 



AIR PASSAGES. 



The condition of the air passages varies from one of perfect freedom in the healthy 

 portions of the lung to a state in which the mucous surface is coated with false membrane 

 or solid exudations of lymph in the diseased parts. By suitable means it is not difficult to 

 isolate the solid white lymph clogging the terminal bronchial tubes and air vesicles in the 

 consolidated tissues, but at a distance from these parts it is only in some cases that a kind of 

 croupy complication exists. I have seen an animal gasping for breath, with its mouth 

 open, nostrils widely expanded, eyes prominent, and visible mucous membranes of a bluish 

 red color : on opening the air passages of this cow after death, they were found throughout 

 their whole extent nearly filled with a deposit similar to that usually found on the surface 

 of the diseased lung. 



There is little necessity for prolonging this description of cadaveric manifestations. 

 The heart s sack is sometimes thickened by deposits around it. Not unfrequently it con 

 tains an excess of serum. The heart itself is contracted and pale, containing a little dark 

 blood. The organs of digestion at different stages manifest a state of dryness. The third 

 stomach, which is so constantly packed with dry food in febrile diseases, is in the same 

 condition in pleuropneumonia. I have known the mucous layers spotted with irregular 

 or circular congestions or blood extravasations, and the membrane softening in these parts 

 has become perforated. In advanced cases there is more or less diffuse redness, and even 

 blood extravasations in the large intestine, with fluid, fetid, and sometimes slightly blood 

 stained, excrement, such as is discharged during life. 



The anaemia or bloodless condition of other tissues the dark, dry look of the meat 

 dressed by the butcher, the yellow color of the fat in some cases, and the small quantity 

 of fat left in animals that have succumbed under a chronic attack, are all general signs of 

 greater or less value when taken in conjunction with the changes occurring in the chest. 



THE CAUSES OF THE LUNG PLAGUE. 



The facts which have been adduced in the foregoing pages would seem sufficient to 

 set at rest discussions as to the causes hitherto alleged as giving rise to the spontaneous 

 development of contagious pleuropneumonia. Nevertheless we have seen that wherever 

 the malady appears for the first time the relation of its undoubted cause and effect is usu 

 ally overlooked. Many circumstances tend to obscure the observations even of experts, 

 and it is more particularly in large cities, where the disease is most common and observers 

 more numerous, that conditions mislead and have misled. With a view therefore to impede 

 the renewal of false theories which have up to the present day insured the steady repro 

 duction and propagation of this bovine pest, it may be well to enter into details under 

 three heads : 



1st. The alleged original causes of the lung plague. 



2d. Contagion and infection. 



3d. Conditions favoring or insuring communication of the disease by actual contact 

 or approach. 



