198 



which the septic bacilli gain admittance into the 

 S3^stems of birds, are two in number : — b}^ way of 

 inoculation and by way of ingestion. The latter is the 

 most usual method of infection, and since therefore 

 the disease when so produced may be regarded as the 

 typical form, it will be best to consider it first in this 

 connection. 



The bird having taken the germs into its alimen- 

 tary tract through the medium of either food or water, 

 a circumstance to which it is of course always liable, 

 these germs are first of all subjected to the action of 

 the digestive ferments, and here comes in another 

 instance of the great law of natural selection. Just as 

 in an arnn' of men there are innumerable differences 

 not only in size and nationality, but also in sturdiness 

 and virility through the influences of heredity, age, 

 feeding, etc., so bacilli of an}^ particular species differ 

 among themselves, not onl}^ in size and shape, but 

 also in virulence, owing certainh' ver\^ often to 

 environment, and probably sometimes to heredit3\ 

 Following this, no doubt in the great majority of cases, 

 if the germs are of an indifferent quality of vitality or 

 virulence, the digestive juices are sufficient in them- 

 selves to end the business at once. If the germs are 

 not of so mild a fibre, and if therefore some of them 

 escape the fate which threatens them in the cavity of 

 the canal, and do succeed in permeating the walls of 

 the blood vessels which ramify througliout the coats 

 of the stomach and intestines, the}' then encounter 

 the antitoxins of the blood (plasma and leucocytes), 

 and the struggle for supremacy begins over again. 



And now the first pathological condition is mani- 

 fested—a more or less violent muco-enteritis in which 

 will be found congestive patches of varying intensity 

 and extent in various parts of the intestinal mucous 

 membranes, with often a discoloration of the contents 

 of the small intestines by disintegrated blood. This 



