CHAPTER XVIII. 



The Morbid Inter-action of Structure and Environment — All Disease Processes 

 probably attended by Structural Change, and therefore Natural Varia- 

 tions — Structural Deficiency. 



Having inquired into the causation of S and E separately, it 

 now becomes necessary to consider these tw6 vital factors in 

 the mutual inter- action of disease, for we have seen that disease 

 is an abnormal inter- action of S and E. It will be remembered 

 that we first inquired into the nature of E, and that the 

 last chapters were devoted to the consideration of the causa- 

 tion of S. We found that S was mainly the outcome of 

 heredity, but that it was also influenced, though in a lesser 

 degree, by E. Accordingly we treated of the causation of S 

 under two heads : Under the one we considered the general 

 principle of heredity ; and under the other the influence of E 

 upon S. So that E has had to be treated of in a twofold 

 manner — ( I ) As one of the two great vital factors ; (2) As 

 a plastic agent. 



We now cease to study the two great factors of disease 

 apart, and proceed to consider their morbid inter-action — 

 namely, disease. 



One of the first things that strikes us regarding the power 

 of E to set up disease is, that it varies widely with the 

 nature of S. This is, of course, as we should expect; for, 

 while speaking of the simple cell, we saw how S + E repre- 

 sents certain material conditions, and how, as a result of such 

 conditions, a definite inter-action takes place. Now, differences 

 of S signify differences of these material conditions, and, in 

 consequence, differences in the result. S may be such that 

 S + a particular E shall call forth healthful inter-action, or 

 such that S + this same E shall issue in morbid inter-action. 

 This truth is abundantly exemplified in the diseases of man. 



