114 BOVINE PATHOLOGY. 



observed that infection, contagion, and inoculation are 

 different only in degree, and directly related to the readi- 

 ness with which a disease is communicable from one 

 animal to another. Thus, most active and freely com- 

 municable disorders are propagated by infection, generally 

 also by contagion, and they may be generated by inocula- 

 tion. In infection the organic particles principally gain 

 entry through the delicate lining membrane of the air- 

 passages. We can easily understand that some of these 

 particles will be able to produce no effect when they come 

 in contact with the surface of the cuticle through which 

 they cannot readily pass, and thus a disease may be infec- 

 tious and not contagious. Again, certain of these organisms 

 may require to undergo development probably on a mucous 

 surface before they can cause disorder when they enter 

 the blood. We do not accordingly find that infectious 

 diseases are always communicable by inoculation, though 

 most of them can be thus propagated with facility. Con- 

 tagion, mediate and immediate, is well illustrated in 

 certain skin diseases, due to animal parasites. Specific 

 disorders affect some animals more readily than others in 

 consequence of predisposition or immunity. Generally, 

 debilitated animals are most prone to disorders of this 

 character, and debilitating influences even after expo- 

 sure to infection or contagion may act as determining 

 causes (causes which render the system susceptible after 

 exposure to the influence of the disease producer) . Shocks 

 to the nervous system are especially potent in this respect. 

 Immunity may be due to conditions of the species (specific), 

 or of the individual. The latter form is especially marked 

 in epizootic or enzootic outbreak when all of the herd will 

 be more or less victimised, except a few animals, which 

 remain quite free from disease. Immunity is some- 

 times secured by a previous attack of disorder as is well 

 seen in cases of smallpox in man, this results from pro- 

 found alterations in nutrition, but it seems that in time 

 this immunity becomes lessened. Again, immunity may 

 be secured by an attack of an allied disorder. When the 

 latter is much milder in its attack it may be artificially 



