CHAP. IX.] NERVOUS AND ELECTRICAL FORCES COMPARED. 241 



forces. We must not, however, omit to notice the following points, 

 in which the analogy does not hold good. In the development of 

 nervous power,, there is nothing, so far as our present anatomical 

 knowledge enables us to decide, resembling that completion of the 

 circuit, which is the necessary prelude to galvanic action, or the 

 interruption of it, which is followed by the cessation of that action. 

 The mental or physical stimulus, which must be regarded as a 

 necessary element in every nervous action, stands in lieu of the 

 former ; but how could it accomplish the completion of a nervous 

 circuit is a question at present involved in the greatest obscurity. It 

 is, indeed, a favourite notion with some, that the looped arrange- 

 ment of the peripheral nerve-fibres, in muscles and on some sentient 

 surfaces, forms part of a nervous arc, which is completed at the 

 centre ; nor is it impossible to conceive a mechanism by which the 

 completion or interruption necessary for the development or the 

 stoppage of the nervous power might be accomplished. But it 

 would be hazardous to speculate on such a subject until anatomical 

 research has revealed to us more information respecting the exact 

 disposition of the elements of the vesicular matter. 



The gelatinous fibres appear to want the provision which we have 

 noticed in those of the tubular kind for insulating the nervous 

 power. They supply the unstriped muscular fibres, which probably 

 require a stimulus less definite, as well as of less intensity, than that 

 necessary to excite and regulate the action of the striped fibres. 

 This difference of character in the conducting fibres is worthy of 

 note in making comparison between the respective modes of action 

 of the nervous and electric forces. 



"We come now to inquire whether, by means of the ordinary tests 

 for electricity mentioned in a former page, we can obtain any evi- 

 dence of the identity of the nervous and electrical forces. The 

 results of experiment certainly afford no support to the advocates 

 of the electrical theory ; and indeed it will be seen that there are 

 difficulties in the way of obtaining the necessary conditions for a 

 satisfactory result, which of themselves invalidate the experiments 

 which have been reported to prove favourable to that theory. 



Attempts have been made to affect the galvanometer by bringing 

 the nerves of living animals into connexion with it. This is done 

 by inserting wires into the exposed nerve, and attaching their oppo- 

 site extremities to the galvanometer. When the nervous power is 

 excited so as to cause muscular contraction, the needle, it is said, is 

 deflected.* The experiment, however, has failed in the hands of 

 * David, quoted by Miiller, p. 685. 



VOL. I. R 



