Medicine, 271 



sense of the impropriety and danger of suddenly 

 going from one extreme of excitement to another/ 



Near the close of the eighteenth century, a new- 

 medical theory was presented to the world by Dr. 

 Erasmus Darwin, in his celebrated work which 

 he entitled Zoonomia. 



According to this theory, there is, in every part 

 of the animal system, a living principle, which is 

 termed Sensorial Power, which is considered as 

 the immediate cause of all its motions, and is sup- 

 posed to be secreted in the brain and spinal mar- 

 row. This sensorial power is capable of beino- 

 acted upon in four different ways, or it possesses^ 

 in other words, four different faculties or modes 

 of action, which, in their passive state, are de- 

 nominated irritability, sensibility, voluntarity, and 

 associability ; and in their active state, or during 

 exertion, they are termed irritation, sensation, vo- 

 lition, and association. The faculty termed irrita- 

 tion is exerted, and produces fibrous motions in 

 consequence of the stimulus of external bodies 

 acting on any part of the system where sensorial 

 power resides. That of sensation is exerted in 

 consequence of the stimulus of pleasure or pain, 

 occasioned by fibrous motions originally produced 

 by the sensorial power of irritation. That of vo- 

 lition is exerted in consequence of the stimulus of 

 desire or aversion, occasioned by fibrous motions, 

 which had been previously produced by the senso- 

 rial power of sensation. That of association is at 

 first exerted in consequence of the stimulus of 

 fibrous motions, previously occasioned by irritation, 

 sensation, or volition. 



Having thus stated the various modes of action of 

 the sensorial power. Dr. Darwin proceeds to de- 

 liver the other fundamental principles of his theory, 



r See Prown's Eltmenttf passiiw. 



