196 THE HUMAN BOD Y. 



them produce very different results if different things were* 

 interposed in their course. In one case the current might 

 be sent through water and decompose it, doing chemical 

 work ; in another, through the coil of an electro-magnet 

 and raise a weight; in a third, through a thin platinum 

 wire and develop light and heat; and so on, the result 

 depending on the terminal organs, as we may call them, 

 of each wire. Or on the other hand we might gen- 

 erate the current in each wire differently, in one by a 

 Daniell's cell, in a second by a thermo-electric machine, 

 in a third by the rotation of a magnet inside a coil, but the 

 currents in the wires would be essentially the same, as the 

 nervous impulses are in a nerve-fibre. No matter how 

 they have been started, provided their amount is the same, 

 whether they shall produce similar or dissimilar results, 

 depends only on whether they are connected with similar 

 or dissimilar end organs. 



The Nature of a Nervous Impulse. Since between 

 sense-organs and sensory centres, and these latter and the 

 muscles, nervous impulses are the only means of communi- 

 cation, it is through them that we arrive at our opinions- 

 concerning the external universe and through them that 

 we are able to act upon it; their ultimate nature is there- 

 fore a matter of great interest, but one about which we 

 unfortunately know very little. We cannot well ima- 

 gine it anything but a mode of motion of the molecules of 

 the nerve-fibres, but beyond this hypothesis we cannot go 

 far. In many points the phenomena presented by nerve- 

 fibres as transmitters of disturbances are like the phenom- 

 ena of wires as transmitters of electricity, and when the 

 phenomena of current electricity were first observed there 

 was a great tendency, explaining one unknown by another,, 

 to consider nervous impulses merely as electrical currents. 

 The increase of our knowledge concerning both nerves and 

 electric currents, however, has made such an hypothesis 

 almost if not quite untenable. In the first place, nerve- 

 fibres are extremely bad conductors of electricity, so bad 

 that it is impossible to suppose them used in the Body for 

 that purpose; and in the second place, merely physical con- 



