l6o THE CAUSATION OF DISEASE. 



structural alteration, and are not, therefore, natural variations, 

 it matters not ; both pathological and physiological changes 

 can yet be brought under the same general principles. I 

 shall, therefore, in this chapter, regard all cases of disease as 

 pathological variations. I do not think I shall greatly err in 

 so doing. 



Diseases, then, belonging, as they do, to the great class of 

 natural variations, will obviously be governed by the same laws 

 as to causation. Just as, in response to specific E's, some 

 organisms will readily take on certain physiological variations, 

 so some organisms will readily take on certain pathological 

 variations. And just as, on the other haud, some organisms 

 refuse to take on certain physiological cbaracters, so some 

 equally refuse to take on certain pathological characters. 



We can no more compel an animal or a plant to vary in 

 any given pathological direction, by subjecting it to different 

 forms of pathogenic E, than we can force on animal or plant 

 any particular physiological variation by exposing it to specific 

 modifications of E. And we shall have no difficulty in believing 

 this, if we keep ever before us the kinship between pathological 

 and physiological variations. 



No doubt we can fracture a skull or inoculate a poison, but 

 who will engage to produce general paralysis of the insane in 

 any given individual (to make no mention of other more subtle 

 forms of mental disease) by applying to him a specific patho- 

 genic E ? Where, again, is the experimentalist that can 

 produce at will such diseases as bulbar paralysis, ophthalmo- 

 plegia interna, primary lateral sclerosis, goitre, haemorrhage into 

 the internal capsule, mediastinal tumour ? Even disorders whose 

 causation is more or less perfectly understood, such as drunkard's 

 liver and granular kidney, do not always occur under the E 

 which is wont to produce them. 



"I do not believe," says Sir J. Paget,* "that through any 

 external conditions whatever, and independent of inheritance, 

 any one can become the subject of cancer, gout, tuberculosis, or 

 any disease allied to them. External conditions may hasten 

 the appearance of such diseases, determine their seat, and 



* " Clinical Lectures/' p. 414. 



