338 Medicine, [Cha?. IV. 



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 Zobnomia^, 



According to this theory, there is, in every part 

 of the animal system, a living principle, which is 

 termed Sensorial Fowei\ 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 being 

 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 deno- 

 minated irritability, sensibility, voluntarity, and 

 associability ; and in their active state, or during 

 exertion, they are termed irritation, sensation, voli- 

 tion, 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 



'^ Dr. Erasmus Darwin was a native of Nottinghamshire, 

 where he was born, December 12, 1/31. He was educated at 

 the university of Cambridge, and graduated Bachelor of MedU 

 cine in that institution in 1755, and soon afterwards commenced 

 the practice of physic at Litchfield, where he long resided in the 

 honourable, useful, and profitable practice of his profession. His 

 first great work, tlie Botanic Garden, was published in 1/89^ 

 the Zomomia in I794j hh Phytologia in 1799? ^^^^ ^^is Temple of 

 Nature a short time after his death, which took place on the 18th 

 of April, 1802. 



