42 DEPARTMENT OF AGRICULTURE. 



point. The voice of the ablest and most careful observers, who have 

 studied i>]euro-pneumonia practically, is unauinious ou the point; and 

 althoufjh in every country the tendency has been at first to regard this 

 insidious disease as originating- from atmospheric agencies, when tlie 

 facts have been probed by skillful men, the earlier opinions have been 

 rejected. Gerlach, in 1835, Delafond, in 1844, and Sauberg, in 1840, 

 publishe very abundant and conclusive testimony on this point. 



THE PATHOLOGY, Oil NATUEE OF THE LUNG PLAGUE. 



There is nothing more dangerous and better calculated to retard inquiry 

 and truth than the common practice of speculating as to the nature of 

 specific diseases in men and animals by the analogical method. Bovine 

 pleuropneumonia has been widely supposed to be an inflammation ot* 

 the lungs, governed by the same conditions that operate in relation to 

 ordinary inflammations of the chest in the human family, and, indeed, 

 in all mammalia. The characteristic signs of small-pox depend on a cuta- 

 neous inflammation, but have apperauces different from the results of a 

 scald. It is as rational to define variola inflammation of the skin as it is to 

 declare that the lung disease of cattle is an inflammation of the air passage 

 and lungs. The local phenomena of the disease are associated with and 

 characterized by inflammatory changes, but the cause in operation 

 inducing all this is peculiar and specific. 



The lung plague is a malignant fever, never generated dc novo, so far 

 as reliable observation has yet reached, dependent on the introduction 

 of a virus or contagion into the system of a healthy animal. This prin- 

 ciple produces a local change if inserted into any part provided with a 

 connective or fatty tissue, in which it most readily penetrates. The same 

 local change is produced by its contact with the delicate mucous surface 

 of the bronchial tubes. It adheres, spreads not unlike cancer, regard- 

 less of the nature and importance of the structure it invades, and 

 traverses the lym])hatic vessels to form deposits in the neighboring 

 lymphatic glands, but not generally throughout the lymphatic sys- 

 tem. At first there is no great intensity of inflammation. Suppuration 

 is only a later complication from the concomitant nonspecific change in 

 masses of areolar or connective tissue. Congestion and a serous 

 infiltration rapidly surround the spot inoculated. Heat, redness, pain, 

 and swelling manifest themselves, aiul the reproduction and extension 

 of the tissue-destroying virus may be judged by the extent of swelling ; 

 the amount of the yellow gelatinous serosity or exudation which fills the 

 lung tissue, thickens white fibrous structures, blocks up the adipose 

 tissue corpuscles out of which the fat is displaced, and is only limited 

 in mauy casi s by the amount of connective tissue it can invade, by gravi- 

 tation or otherwise, and the endurance of the animal under a process so 

 ]>rostrating sind d('])l('tive. 



Tliat all Ihis happens, we have tested by experiment. A susceptible 

 auimaris inoculated in the dewlap, and at the expiration of a week or 



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