148 RELATIONS OF BACTERIA TO DISEASE. 



those which act on the nerve fibres and cells. Further, 

 it may be again stated that the action of the products 

 circulating in the system is often manifested more by 

 symptoms than by tissue changes, though our knowledge 

 of the latter, especially in the nervous system, is gradually 

 being extended. 



(i) Local Lesions. By this is meant the changes pro- 

 duced in the neighbourhood of the bacteria. These 

 changes are, on the one hand, of the nature of inflam- 

 matory reaction, from the most intense vascular changes 

 in acute inflammations to the more or less chronic pro- 

 liferative changes especially of connective tissue, and, on 

 the other hand, of the nature of cell-poisoning, leading to 

 degeneration or necrosis, especially of the more highly- 

 developed elements. They may be roughly classified as 

 acute and chronic changes. As already pointed out, the 

 effects of a given organism vary in different animals, and 

 further, where the lesion is approximately the same in 

 different animals, differences in their minor characters 

 may be found. Examples of this latter are furnished in 

 the case of tubercle. 



Position of 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 conditions 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 

 primary 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 speci- 

 ally liable to be affected in this way vary in different 

 diseases. Pyogenic cocci show a special tendency to 

 settle in the capillaries of the kidneys and produce miliary 

 abscesses, whilst these lesions rarely occur in the spleen. 



