APPENDIX E. CXI 



with respect to some of these diseases, which have been 

 hitherto almost wholly ignored. 



In the consideration of the nature and causes of disease, 

 we have always to keep in mind two principal sets of factors. 

 Each person exists with structural characters and functional 

 properties which, though in the main similar to those of his 

 fellow-men, have nevertheless individual peculiarities more 

 or less marked. These may be inherent or acquired, habi- 

 tual or occasional. Amongst these individual peculiarities 

 are ranged what time-honoured custom has called the 'pre- 

 disposing causes ' of disease. On the other hand, man, with 

 his individual peculiarities, lives in a world of change, exposed 

 to the incidence of constantly varying external conditions, 

 which, acting upon individual peculiarities, or upon the aver- 

 age human nature, become, in proportion to their deviation 

 from the usual condition of things, so many ' exciting causes ' 

 of disease. 



These two sets of factors must never be lost sight of. In 

 the majority of cases, both come into operation in the pro- 

 duction of the resultant morbid condition, although in others 

 one or other of them alone may seem to be so potent as of 

 itself to determine the morbid manifestation. The person 

 who inherits a tendency to destructive lung disease may de- 

 velop this morbid condition under the influence of exciting 

 causes which would scarcely affect another who inherits no 

 similar weakness (predisposing cause). On the other hand, 

 just as contact with boiling water, owing to the exceeding 

 potency of the ' exciting cause,' may determine a lesion of 

 the skin in any individual (quite independently of the exist- 

 ence of a ' predisposition'), so may a person who is born with a 

 weak and unstable nervous system become insane or epileptic, 

 independently of the influence of any obvious exciting causes. 



All diseases are, in fact, due to altered structure or mole- 

 cular composition, whether visible or invisible, ascertainable 



