THE CAUSATION OF DISEASE. 1 69 



conspire to the production of an effect are alike causes, alike 

 agents, and that there is, in most cases of insanity " (I should 

 have preferred to say " all cases"), c: a concurrence of con- 

 ditions, not one single effective cause. Mental alienation often 

 appears as the natural issue of all precedent conditions of life, 

 mental and bodily, the outcome of the individual character as 

 affected by circumstances ; in such case, the germs of the 

 disease may have been latent in the foundation of the character, 

 and the final outbreak is but the explosion of a long train of 

 antecedent preparation. In vain, then, is it to try to fix 

 accurately upon a single cause, moral or physical/' 



In the above symbolical formulae, E has always referred to 

 the external environment ; but in certain cases of disease this 

 letter may conveniently symbolize the share taken by local 

 bodily change in provoking disordered action in another part 

 of the body. We have seen that diseases rarely remain strictly 

 localized ; that, owing to the mutual dependence of tissue upon 

 tissue, disease in one part is very apt to set up morbid action in 

 others. Now, it often happens that the most obtrusive, and, in- 

 deed, the most important, morbid action is not the primary oue, 

 but some morbid action secondary to it. Thus, the local 

 trouble of indigestion, or the irritation of a worm in the bowel, 

 may excite an epileptic fit ; thus also, gastric disturbance or 

 bronchitis may bring on an asthmatic paroxysm ; and, in like 

 manner, a decayed tooth may cause neuralgia. 



In considering the causation of disorders thus arising, we may, 

 then, conveniently regard the central erring tissue as an indepen- 

 dent S, and the peripheral irritation as an E acting thereon, and 

 in all such cases we may lay down the general principle that the 

 amount of peripheral irritation needful to call forth the abnormal 

 central state is in inverse proportion to the abnormal tendency 

 of the centre. The latter may, as it were, be so fully charged 

 with dynamite, that it will explode spontaneously : as when 

 an asthmatic or epileptic fit occurs though every care be taken 

 to prevent any peripheral irritation. On the other hand, the 

 morbid tendency may be so slight that a very intense peripheral 

 irritation is requisite to bring out the morbid action. In such 

 a case the causation may be numerically expressed thus : 



