2l8 



into living tissues, must not onl}' exercise some 

 influence on these tissues but must also themselves 

 undergo some furtlier change. First of all the same 

 coagulation of the fibrinogen lakes place in them as 

 has been previous!}^ noticed to occur here and there in 

 the blood of the vessels, and which was itself the 

 mechanical cause of the extravasation. The fibrinous 

 clot which is thus formed then undergoes a form of 

 gangrene which is called coagulation necrosis, and in 

 which the cells become completely disorganised and 

 are replaced by a granular, structureless material. 

 This partly constitutes the caseation, but this latter is 

 also in great part due to the massing together of 

 countless numbers of the bacilli themselves. (Many 

 indeed of the smaller nodules are seen to consist 

 entirely, or at any rate almost entirely, of bacilli with 

 the blocked vessel for a centre. When of this 

 character they are usually rosette shaped, and this 

 may be regarded as fairly typical of them in their 

 earliest stages.) Gradually, b\' a mixture of processes, 

 i.e. multiplication of bacilli, coagulation necrosis of 

 the clot, and necrosis of the surrounding zone of 

 tissues through pressure and consequent cutting off 

 of the blood supply, they become larger and larger, 

 until from merely containing numerous small nodules 

 whole areas of the affected organ often become large 

 inasses of cheesy material simph^ through coalescence 

 of these nodules. Of course this development de- 

 pends entirely on the length of time the bird survives. 

 We have seen that sometimes it dies before there has 

 been sufficient time for the production of any nodules 

 at all, and at the other end of the scale I have fre- 

 quently found as much as two-thirds of the enlarged 

 spleen or liver to be composed of nothing but this 

 cheesy material. 



It will have been gathered that these two organs 

 are of course the most usual sites, but they are not 



