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composition and constitution of the blood ; the white corpuscles 

 may be destroyed, or their movements enfeebled, and, by any 

 such changes, the blood, which previously could resist the attacks 

 of the various parasitic micro-organisms, is so modified as to 

 render it a suitable soil in which these organisms may develope 

 and thrive. This view is confirmed by the experiments of Ross- 

 bach and Rosenberger, who found that when papain or sterilised 

 septic blood was injected into the vessels, micrococci developed 

 in the blood with extraordinary rapidity. The blood, that is to 

 say, was so modified in character that the micrococci which could 

 gain no entrance previous to the injection of papain, afterwards 

 found a congenial soil for their development. 



We may take another illustration from the process of fer- 

 mentation. By fermentation sugar can be converted into car- 

 bonic acid and alcohol ; but a pui-e saccharine solution does not 

 ferment on the addition of a small quantity of fungi or bacteria 

 in a pure state. Some nitrogenous material must be added as 

 well, to act as food to the organisms, and then the transformation 

 into carbonic acid and alcohol commences. 



That it is only in particular conditions of a tissue, or at special 

 times in its growth, that it furnishes suitable soil for the growth 

 of the bacilli is shown by the following illustration from the 

 vegetable world, furnished by de Bary : — 



"The common garden-cress Lepidium sativum, is often at- 

 tacked by a parasitic fungus of comparatively large size, Cystopus 

 candidus. In consequence of this it shows considerable degenera- 

 tion, swellings, curvatures of the stem, and often also of the 

 fruits, and on these parts and on the leaves white spots and 

 pustules subsequently turning to dust, which are formed by the 

 sporogenous organs of the Cystopus, and give the entire pheno- 



