THE DISEASES OF CATTLE. 39 



ficient ventilation, filth, insufficient and bad food, may indeed 

 predispose to the disease, concentrate the animal effluvia, and 

 become the matrix and nidus of the organic poison ; but still, 

 not one, alone, of these circumstances, or even all of them 

 combined, can produce the disease in question. There must 

 be the subtle poison to call them into operation, the specific 

 influence to generate the disease. 



" On 'the other hand, it appears probable that the exciting 

 cause, whether it be contagion, or whatever else, cannot, of 

 itself, generate the disease y but that certain conditions or pre- 

 disposing causes are necessary to its existence, and without 

 which its specific effects cannot be produced. But although 

 these remote or predisposing causes are very numerous, they 

 are often difficult of detection ; nay, it is sometimes inapossible 

 to tell to what the disease is referable, or upon what weak 

 point the exciting cause has fixed itself. A source of per- 

 plexity results fi'om the fact. . . . The predisposing 

 causes of the disease admit of many divisions and subdivisions ; 

 they may, however, be considered under two general heads — 

 hereditary and acquired. 



" With reference to the former, we know that good points 

 and properties of an animal are transmitted from one genera- 

 tion to another ; so also are faults, and the tendencies to par- 

 ticular diseases. As in the same families there is a similarity 

 of external form, so there is an internal likeness, which 

 accounts for the common nature of their constitution, modified 

 however, by difference of age, sex, etc. 



" Among the acquired predisposing causes of pleuro-pneu- 

 monia may be enumerated general debility, local weakness, 

 resulting from previous disease, irritants and stimulants, ex- 

 posure to cold, damp, or sudden changes of temperature, the 

 want of cleanliness, the breathing of an atmosphere vitiated by 

 the decomposition of animal or vegetable matters, or laden 

 with any other impurity. In short, under this head may be 

 included every thing which tends to lower the health and 

 vigor of the system, and consequently to increase the suscep- 

 tibility to disease. 



