22 PATHOLOGICAL MYCOLOGY. 



significance of medical infective diseases, and the one basis on which 

 a therapeutic system can be constructed. The best researches on the 

 subject lead us to believe that normal healthy tissues are perfectly 

 free from the presence of micro-organisms, and their presence thus 

 directly indicates a diminution in the vitality of the tissue elements 

 near which they lie. Fully vitalised tissues react so powerfully on the 

 micro-organisms, or their germs, as to prevent any development on 

 their part ; but this condition once lost, the tissues are no longer 

 resistant, but supply all the requirements of the parasite in the points 

 most closely essential to its life and growth, and become, for the time 

 being, a delicately adapted incubator, in which the life of the organism 

 passes rapidly through its phases. 



Conditions necessary to Investigation. 



20. In the investigation of any disease where one is led to suspect the 

 interference of a micro-organism, there are certain points of essential 

 importance which must be clearly proved before any such proposition 

 can be established as that a micro-organism is directly concerned in 

 its causation. 



(a.) Firstly,'we must have a well defined type of disease, accompanied 

 in all cases by symptoms so characteristic as to constitute a specific 

 malady. In all such cases, at one stage or other of their course, it 

 must be possible to demonstrate a micro-organism of constant form, 

 and disposed in a constant relation to the blood or solid tissues of 

 its host ; and this organism must only be found in the tissues of 

 animals which are suffering, or have suffered, from the distinctive 

 symptoms already observed. 



{l>.) Secondly, by carefully effecting the isolation of these organisms 

 from the tissues, and inoculating them in nutrient media specially 

 adapted for their support, it must be possible to cultivate them arti- 

 ficially apart from the tissues of their host. These cultivations must 

 be absolutely shielded from contamination by the various extraneous 

 conditions with which they may come into relation, and must re- 

 tain the same typical appearances throughout a long series of culti- 

 vations carried on under constant conditions ; showing that they have 

 received no new elements from without, nor has any change occurred 



