116 BOVINE PATHOLOGY. 



SO on. Often we have to deal with complications of these 

 disorders, non-specific conditions to which they give rise, 

 thus in eczema to sloughing of the hoofs and mammitis ; 

 but such especially occur in the less acute disorders which 

 we shall have to mention, cancer and tubercle. It is then 

 that we must be careful to distinguish between primary 

 and secondary disease, for this will considerably affect our 

 diagnosis and treatment. 



It seems that an outbreak of a specific disorder, espe- 

 cially when the disease is epizootic, may be related to the 

 severity of all exciting causes. Thus, if the disease 

 first appears in a place where from neglect of sanitary 

 precautions and the general conditions of health, animals 

 are very predisposed, severe attacks and extensive spread- 

 ing power are the results; but if it be unable to gain a 

 firm hold on its first victims, it will never " gain head '' 

 enough to assume a marked degree of virulence. The 

 tendency of disorders of this kind fortunately is to lessen 

 in intensity with time, a happy condition, whereby they 

 are prevented from annihilating races of animals. Some 

 individuals escape by immunity, others by recovery after 

 comparatively slight attacks, and some by accidental or 

 intentional escape from disease-bearing influences. Thus, 

 an outbreak of a disease of this kind passes over a dis- 

 trict, and spreads from it into a new locality, diverging 

 from the centre in every direction, or in lines determined 

 by favorable conditions ; finally, its spread is checked by 

 dilution of the original stock of virus, or by influences 

 unfavorable to its progress as reaching the sea, or a range 

 of high mountains, or being subjected to a frost. The 

 question which next suggests itself to us is the source of 

 these disorders. This question is one of the most urgent 

 of the present day. Either they can originate de novo, or 

 can be propagated only from centres of disease already 

 existing. The latter method of origin of an epizootic 

 affection is well illustrated in those disorders of which 

 rinderpest is the type. It constantly exists under a com- 

 paratively mild form as an enzootic in certain localities as 

 the steppes of Russia and Central China. From these it 



