240 INNERVATION. [CHAP. ix. 



manner, we can insulate the galvanic current by covering the 

 conducting wire with silk, or some other non-conductor, and thus 

 cause it to pass through an indefinite length of wire disposed in 

 coils, or through any number of separate wires disposed parallel to 

 each other, which may be brought into connexion with the metals. 



These remarks are clearly most applicable to those nervous ac- 

 tions which emanate from a centre. But in those in which the 

 nervous force is propagated to a centre, as when pain is excited 

 by touching a nerve, or in the excitation of the motor nerve of an 

 amputated limb by artificial stimuli, the analogy of the mode of its 

 development with that of the galvanic force is not so obvious. 

 Still, when we remember how easily thermo-electric currents may be 

 excited by the disturbance of the equilibrium of heat in a wire of 

 even a single metal, it seems not unreasonable to refer this excita- 

 bility of nerve to some similar proneness to change in it. 



Nothing is more certain than that a very slight mechanical or 

 chemical stimulus to a nerve, whatever be its proper vital endow- 

 ment, is capable of producing in it that state of polarity on which 

 we suppose the manifestation of nervous force to depend : and it 

 seems not incorrect to imagine that, in the battery, the point of 

 departure of the galvanic action may either be at the poles or at 

 the battery itself, according to the place at which the completion of 

 the circuit takes place ; thus affording a more marked analogy to 

 the two kinds of nervous actions above referred to. Thus the con- 

 ducting wires may be in contact with each other, and with their 

 respective metals ; bat, if there be no fluid interposed, there is no 

 action. The instant the fluid is added, the current begins ; and 

 in this case its point of departure may be regarded to be from the 

 battery in analogy with those nervous actions which proceed from 

 the centre. On the other hand, the arrangements of the battery 

 itself may be perfect, but action will not begin until the circuit 

 is completed by bringing the conducting wires into contact. In 

 this case, the polar change may be said to commence at this 

 point of contact, and to travel to the battery, in a manner analo- 

 gous to that in which nervous action is propagated from the peri- 

 phery to the centre. In both cases, moreover, so long as galvanic 

 action continues, the whole apparatus is in the polar state : and so 

 long as nervous action continues, the particular nervous apparatus 

 involved (vesicular matter and nerve-fibre) must be considered to be 

 in a state of polarity through its whole extent. 



Thus far we remark unquestionable analogy in the mode of 

 development and of propagation of the electrical and nervous 



