510 INDUCTION. 



siology (greatly and rapidly progressive as it now is) is embar- 

 rassed by greater natural difficulties, and is probably susceptible 

 of a less degree of ultimate perfection, than even the social 

 science ; inasmuch as it is possible to study the laws and ope- 

 rations of one human mind apart from other minds, much less 

 imperfectly than we can study the laws of one organ or tissue 

 of the human body apart from the other organs or tissues. 



It has been judiciously remarked that pathological facts, 

 or, to speak in common language, diseases in their different 

 forms and degrees, afford in the case of physiological investi- 

 gation the most valuable equivalent to experimentation pro- 

 perly so called ; inasmuch as they often exhibit to us a definite 

 disturbance in some one organ or organic function, the remain- 

 ing organs and functions being, in the first instance at least, 

 unaffected. It is true that from the perpetual actions and re- 

 actions which are going on among all parts of the organic 

 economy, there can be no prolonged disturbance in any one 

 function without ultimately involving many of the others ; 

 and when once it has done so, the experiment for the most 

 part loses its scientific value. All depends on observing the 

 early stages of the derangement ; which, unfortunately, are of 

 necessity the least marked. If, however, the organs and func- 

 tions not disturbed in the first instance, become affected in a 

 fixed order of succession, some light is thereby thrown upon 

 the action which one organ exercises over another : and we 

 occasionally obtain a series of effects which we can refer with 

 some confidence to the original local derangement; but for 

 this it is necessary that we should know that the original 

 derangement was local. If it was what is termed constitu- 

 tional, that is, if we do not know in what part of the animal 

 economy it took its rise, or the precise nature of the disturb- 

 ance which took place in that part, we are unable to determine 

 which of the various derangements was cause and which 

 effect ; which of them were produced by one another, and 

 which by the direct, though perhaps tardy, action of the 

 original cause. 



Besides natural pathological facts, we can produce patho- 

 logical facts artificially ; we can try experiments, even in the 



