CHAP. IX.] NERVOUS AND ELECTRICAL FORCES COMPARED. 239 



Further tests of the presence of galvanic action are found in 

 the magnetization of a steel needle placed within a coil of wire 

 through which the current is made to pass ; and in the evolution of 

 heat and light, which takes place when the circuit is completed or 

 broken. This latter effect, however, does not occur from currents 

 of very feeble intensity. 



Let us inquire how far the phenomena of the nervous force cor- 

 respond with those of this current electricity, and whether it will 

 respond to any of the tests just described. 



It has already been remarked, that the instantaneousness with 

 which nervous power is developed, when a mental or physical 

 stimulus is applied to a nerve, resembles remarkably the rapid evolu- 

 tion of the galvanic force throughout the whole circuit, the instant 

 the necessary contacts are completed. And both cease with equal 

 rapidity when the conditions for their production are destroyed. 



Some analogy is apparent in the conditions which are requisite 

 for the development of both forces. The dissimilar metals and 

 the interposed fluid, which we have seen to be necessary for the pro- 

 duction of the galvanic force, may have as analogues for the develop- 

 ment of the polarity of nerves, the two kinds of nervous matter (the 

 vesicular and fibrous), and the blood. Nervous power is never 

 developed from a centre without the conjoint action of these two 

 kinds of nervous matter. The analogy fails, however, when we 

 compare the relation of the metals in the battery with that of the 

 gray and white matter in the nervous centre. The former need 

 not have any connexion but such as a conducting wire of ever so 

 minute dimensions passing from one to the other is capable of 

 effecting ; that is to say, union of a few points of one metal with 

 as many of the other, is sufficient for the generation and transmis- 

 sion of the polar state. In the nervous centres, however, the points 

 of contact are probably most numerous. The vesicles of the gray 

 matter certainly are brought most extensively into connexion with 

 the nerve-fibres ; and there is much to justify and confirm the 

 opinion that each fibre is connected with a vesicle, and that each 

 vesicle, at least of the caudate kind, may be regarded as the point 

 of departure of one or more fibres. If such an arrangement exist, 

 we may regard each nerve-vesicle, and the fibres emerging from 

 it, with the blood-vessels which play around it, as a distinct appa- 

 ratus for the development of nervous polarity. 



There appears to be a provision for the insulation of the central 

 axis of each nerve-fibre in the white substance of Schwann ; but 

 there is no such arrangement for insulating the vesicles, In like 



