4 PATHOLOGICAL MYCOLOGY. 



under certain abnormal conditions. The invasion of the living body 

 by micro-organisms is a subject which presents great difficulties as to 

 its rationale, and as to the exact manner in which it takes place. 

 In regard to the factors which seem to militate against their entry, 

 Lister has pointed out that the chief peculiarity of the tissues, in this 

 direction, is their vitality. In normal conditions the ingress and pro- 

 gress of micro-organisms are checked by the direct vital reaction of 

 the tissue elements themselves. He further insists that not only is 

 this the case as regards healthy surfaces and the deeper tissues, but 

 that two perfectly healthy mucous membranes or fresh cut surfaces, 

 when brought into close apposition, will prevent the development be- 

 tween them, and even the entrance of micro-organisms — an explana- 

 tion consequent upon the phenomena observed in the urethra, and in 

 connection with healing by first intention. Tissues, then, of what 

 may be termed a Standard Vitality, resist perfectly the entrance of 

 the invading organisms ; but let this standard be but to the least 

 extent lowered, and the chance of an invasion at once becomes great. 

 The question is one of intricate physico-chemical relations between the 

 organisms and the tissue elements of their host, which we cannot at 

 present define more exactly than in the preceding general terms. 

 But in looking; to the conditions and duration of the invasions them- 

 selves, many other points can be established. If it be admitted that 

 this theory of Standard Vitality is correct, it follows, that under normal 

 conditions these minute organisms would be continually excluded from 

 the tissues of the body; but, unfortunately for us, there are acting con- 

 stantly certain very powerful allies of these micro-organisms, through 

 whose aid the vitality of the tissues is lowered. The general vitality 

 of the body may be depressed by cold, want of food, want of sleep, 

 and irregular habits. Even fear, anger, or other strong emotion may 

 act in the same way by altering both secreting tissue and secretions, 

 thus interfering with the proper nutrition of the tissues, and lowering 

 their vitality. 



Means of Ingress. 



4. Further, in the altered secretions, micro-organisms normally 

 present maybe able to multiply much more rapidly than in the normal 

 secretions, or certain varieties which are not usually present, or are 



