140 THEORIES 0F THE NERVOUS POWER. 



cles to which they are distributed, are violently convul- 

 sed. The fact is, that though the impressions are trans- 

 mitted to the brain, the sensorial power being destroyed 

 or suspended, does not furnish the stimulus of volition, 

 nor produce sensation. In sleep we see some of the 

 nervous functions suspended ; while the sensorial are 

 in full play &nd activity. We think, feel and will, but 

 not a muscle obeys the stimulus of volition, because the 

 nervous power does not transmit it. 



Emily. I am satisfied now, perfectly. But it ap- 

 pears to me, you have neglected to say any thing of the 

 manner in which impressions and volitions are commu- 

 nicated by the nerves, or in other words perhaps, the na- 

 ture of the nervous power. This seems to me to be the 

 most interesting part of the subject. 



Dr. B. And it is as obscure, as it is wonderful and 

 curious. Still, little as we actually know about itj there 

 has been no lack of theories to explain its nature, and 

 shed broad daylight upon the dark obscurity. By one 

 of these theories, it was supposed that the nerves had the 

 power of vibration, anH the word nerve itself, which 

 means a tense cord, shows how common this opinion 

 was among the Greeks. This theory, which for a long 

 time had fallen into disrepute, was revived during the 

 last century, with modifications and improvements, by 

 Dr. Hartley. It is unaccountable how such a theory 

 should have ever prevailed, for we can conceive of no 

 part of the animal structure so entirely incapable of the 

 vibratory motions of a tense string, as the delicate, inelas- 

 tic nerves completely enveloped in the surrounding soft 

 parts. By another theory, the nerves were regarded as 

 capillary tubes which convey a fluid secreted by the 

 brain, and called the nervous fluid or animal spirits. It is 

 by the oscillations of this fluid, that impressions are com- 

 municated to the brain, and volitions to the nerves. It 

 has been a very common doctrine among medical men, 

 and seems now to be an article of popular belief. It is 

 enough to say of it however,- that the two essential facts 



