Contagious and Episootic Diseases. 49 



glands, to block these passages to a greater or less extent, 

 and to derange healthy processes by obstructing the flow of 

 blood or lymph or by irritating the parts and producing 

 local inflammation. 



These may serve as principles in the light of which to 

 consider the various theories of the mode of operation by 

 whicli a first attack gives immunity from a second. Fom- 

 hypotiieses have been advanced to account for this immunity, 

 which may be considered seriatim. 



a. The Exha/ustion Theory. This assumes that in the 

 susceptible animal the disease-gerra finds its appropriate 

 food, which has been accumulating from birth, that it uses 

 up this and is starved to death when this supply has be- 

 come exhausted. The theory holds that the presence of 

 the living germ in the system causes the fever, that the 

 fever subsides when the germ dies, and that the disease 

 cannot again recur in the same animal because all the 

 food of the disease-germ which it contained has been used 

 up. This view was naturally adopted by Pasteur, whose 

 chemical experience with beer and wine had accustomed 

 him to gauge the growth of the yeast by the amount of 

 sugar in the malt or grape-juice. It is, however, utterly 

 untenable as applied to the growth of a disease-germ in 

 an animal body. In the animal system the disease-germ 

 lives in a medium which is constantly changing, new food 

 material is taken in several times a day, this new food 

 is being continually built up into living tissues, and from 

 the living tissues so constructed waste materials are being 

 constantly abstracted and carried out of the body. The new- 

 born animal readily contracts a contagious disease, though 

 the whole period of its pre-existence from its inception in 

 the ovum does not exceed one month to one year in the dif- 

 ferent domestic animals ; yet, after a first attack, it may live 

 for many years exposed at frequent intervals to the same 

 contagion, and never again submit to its malign influence. 

 4 



