166 RELATIONS OF BACTERIA TO DISEASE. 



the direct result of damage to the endothelium of the vessels. 

 It may also be pointed out that the various changes referred to 

 are none of them peculiar to bacterial invasion ; they are exam- 

 ples of the general laws- of tissue change under abnormal con- 

 ditions, and they can all be reproduced by chemical substances 

 in solution or in a particulate state. What constitutes their 

 special feature is their progressive or spreading nature, due to 

 the bacterial multiplication. 



(i) Local Lesions. In some diseases the lesion has a special 

 site; for example, the lesion of typhoid fever and, to a less 

 extent, that of diphtheria. In other cases it depends entirely 

 upon the point of entrance, e.g. malignant pustule and the con- 

 ditions known as wound infections. In others, again, there is 

 a special tendency for certain parts to be affected, as the upper 

 parts of the lungs in tubercle. In some cases the site has a 

 mechanical explanation. 



When organisms gain an entrance to the blood from a pri- 

 mary lesion, directly or by the lymphatic system, they may 

 become destroyed, or they may settle in certain organs and 

 produce their characteristic effects. The organs specially 

 liable to be affected in this way vary in different diseases. 

 Pyogenic cocci show a special tendency to settle in the capil- 

 laries of the kidneys and produce miliary abscesses, whilst these 

 lesions rarely occur in the spleen. On the other hand, the 

 nodules in disseminated tubercle or glanders are much more 

 numerous in the spleen than in the kidneys, which in the latter 

 disease are usually free from them. The important point is that 

 the position of the disseminated lesions is not to be explained 

 by a mechanical process, such as embolism, but depends upon 

 a special relation between the organisms and the tissues, which 

 may be spoken of either as a selective power on the part of the 

 organisms or a special susceptibility of tissues, possibly in part 

 due to their affording to the organisms more suitable conditions 

 of nutriment. Even in the case of the lesions produced by dead 

 tubercle bacilli, a certain selective character is observed. 



Acute Local Lesions. The local inflammatory reaction pre- 

 sents different characters in different conditions. It may be 

 accompanied by abundant fibrinous exudation, or by great 

 catarrh (in the case of an epithelial surface), or by haemorrhage, 

 or by oedema; it may be localised or spreading in character; it 



