32 DEPARTMENT OF AGRICULTURE. 



casual agencies capable of inducing the disease, is almost sufficient to establish the purely 

 contagious nature of the disease, but there are several important proofs that deserve 



o 



mention. 



It is seen in all countries where the lung plague appears, that it spreads in proportion 

 to the opportunities of contagion. It is worst in -large cities, where cow feeders have to 

 make frequent purchases. It is apt to diminish in severity as, per example, in the city 

 of Washington, in Dublin, Ireland, and elsewhere so long as the cows are confined to 

 stables in the winter and different herds have no chance of approach. When spring and 

 fine weather arrive, and the cows are turned out during part of the day, or altogether, on 

 commons, parks, or pastures, the presence of any infection results in the rapid dissemina 

 tion of the disease. I had special occasion to study this among the cows turned out into 

 the Phoenix Park, Dublin, and on the commons near Newcastle, in England. 



In 1862 I chose a large estate in Perthshire, presenting the feature of being cut up 

 into farms, on some of which cattle were wholly bred; whereas on others purchases had 

 occasionally been made. The result was the demonstration of the fact that the disease 

 appeared only where it was carried by diseased cattle. The estate was that of Lord 

 Willoughby d Eresby, comprising twenty-six farms, on eleven of which the disease was 

 at different times imported; whereas on the fifteen other farms, interspersed between 

 eleven, the only report to be obtained was, &quot;Never had the disease. Breeds his own 

 stock.&quot; 



A similar inquiry relating to the parish of St. Martin, in Perthshire, showed that 

 pleuropneumonia had appeared there in 1845. Since then ten farms have been visited by 

 the disease, and in every case the attack has been distinctly traced to contact with diseased 

 cattle. Nineteen farms on which cattle are bred and purchases rarely made have enjoyed 

 a perfect immunity. 



The high-prized herds of England, which have been carefully isolated by their pro 

 prietors, have always remained free from the disease, and short-horn breeders have, in many 

 instances, exercised the greatest care not to have any admixture with strange animals, 

 which would certainly have destroyed their stock. 



It is needless to enter at length into the subject of authorities on this point. The 

 voices of the ablest and most careful observers, who have studied pleuropneumonia prac 

 tically, are unanimous on the point ; and although in every country the tendency has been 

 at first to regard this insidious disease as originating from atmospheric agencies, when the 

 facts have been probed by skillful men the earlier opinions have been rejected. Gerlach, 

 in 1835, Delafond, in 1844, and Sauberg, in 1846, published very abundant and conclu 

 sive testimony on this point. 



THE PATHOLOGY OR NATURE OP 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 of the lungs, governed by the same conditions that operate in rela 

 tion to ordinary inflammations of the chest in the human family, and, indeed, in all mam 

 malia. The characteristic signs of small-pox depend on a cutaneous inflammation, but 

 have appearances different from the results of a scald. It is as rational to define variola 



