8 IRRITABILITY 



of one organ upon another. Only as a result of the continual 

 action of stimuli is life maintained, in that the stimuli produce 

 continual "excitement" in the irritable substance. The degree of 

 irritability differs in various plants, animals, in different struc- 

 tures of the body, and even in the same individual at differ- 

 ent times under different circumstances. The strength of the 

 "excitement" depends on the one hand upon the degree of irri- 

 tability, and on the other upon the strength of the stimulus. The 

 irritability itself is influenced and changed by the action of the 

 stimuli. If the stimuli are too strong and are of prolonged dura- 

 tion, the irritability diminishes as a result of exhaustion ; if weak 

 stimuli act during a prolonged time, the irritability increases. 

 The healthy organism has a mean degree of irritability. Disease 

 occurs when this state is altered by strong stimuli or by an 

 absence of stimulation. Disease and health, therefore, differ not 

 qualitatively but only quantitatively. It is here seen that we have 

 the first attempt at a systematic interpretation of the effects of 

 stimulation, and it is astonishing how sharply and successfully 

 Brown has pointed out the foundations of this important field. 

 He has in this way not only amply compensated for the great set- 

 back in the history of the teaching of irritability produced by the 

 confusions of conceptions created by Haller and the vitalists, but 

 also placed the whole of the physiology of stimulation on a firm 

 foundation upon which it is possible to build further. Though it 

 is true that many of his special theories, in particular those on 

 nature and the origin of disease, are quite erroneous, still a just 

 critic must judge work in relation to the period in which it was 

 written, and I question if at the present day the science of medi- 

 cine does not contain teachings which in a hundred years will 

 also prove untenable. 



Johannes Miiller (1801-1858) then added an important stone 

 to the building up of our knowledge of irritability. This was the 

 clear recognition of the specific energy of living substances. We 

 have already found the germ in Borden's term "sensihilite propre" 

 or "sensihilite particuliere." Brown was also of the opinion that 

 different living objects possessed different types of irritability 

 and that excitation of their special functions was not dependent 



