TISSUE-SELECTION IN THE GENESIS OF DISEASE. 837 



condition which most concerns us is that of a favorable nidus. A 

 favorable nidus is one in which the germ is enabled to carry on its full 

 vital functions, and to propagate its kind. It is therefore manifest 

 that the constitution of the nidus must be free from elements antago- 

 nistic to such vital functions. It must not be too hot, or too cold, or 

 too dry, and its chemical constitution must be favorable. Experiment 

 has proved that oxygen in excess is deleterious to these organisms ; its 

 absence, entire or partial, will then be one of the requisites in this 

 chemical constitution of the nidus. Is not such nidus best found in de- 

 caying or degenerating animal tissue ? Where there is full vital ac- 

 tivity in any animal tissue, the blood which nourishes it keeps it duly 

 supplied with oxygen. In healthy tissue, then, we have a condition 

 unfavorable ; but when from some cause the nutrition of the tissue is 

 interfered with and a condition of degeneration is induced, this antago- 

 nistic element is removed or at least diminished, and the tissue affords 

 the nidus favorable to the vital phenomena of the germs. These vital 

 phenomena are perhaps best studied in the analogous case of the spores 

 of yeast {torula cerevisice). "When this, which is to all intents and 

 purposes a germ, is placed in favorable circumstances, its activity 

 commences, it rapidly multiplies and gives rise to changes in the 

 surrounding material. In this case we call the process " fermentation." 

 A germ, bacillus, bacterion, or vibrio, when placed in relation to tissue 

 which affords a favorable nidus, assumes its full vital activity; it mul- 

 tiplies and gives rise to changes in the tissue with which it is in con- 

 tact. These changes we call " inflammation." In fact, it would appear 

 that these germs in one sense fulfill the part of Nature's scavengers, 

 and by setting up inflammatory changes in degenerate tissues lead to 

 their removal. Be this as it may, the diseases to which the germs 

 give rise are all more or less of an inflammatory nature. 



Thus, then, it would seem that one of the chief vital functions of 

 these germs is to excite an inflammatory process in degenerate tissue. 

 Is it not conceivable that germs may have existed, or even do still 

 exist now, whose function is strictly limited to action on degenerate 

 tissue ? that this may perhaps have been the limit strictly assigned to 

 them ? Let us suppose this to be the case, and see how, from this 

 limited condition, germs have acquired power to overstep these limits, 

 and thus to give rise to the protean aspects of disease that we now 

 meet with. What has caused variation in the animal world but the 

 influences of surrounding circumstances ? In the relationship of the 

 germs and the degenerate tissue, it is plain that in one sense the germs 

 are the active, and the tissue the passive, elements. But, looked at 

 from the tissue point of view, it will also appear that this passive con- 

 dition possesses considerable indirect influence on the germs ; that, in- 

 deed, " passive " is hardly the word to express the action which must 

 largely modify their constitution. It would be impossible for the 

 germs to live, to grow, to multiply on a certain tissue without be- 



