NERVE 601 



merely an excitation of each nerve-element by the one next 

 it, as some have supposed. 



Double Conduction. When a nerve (or muscle) is stimu- 

 lated artificially, the excitation runs along it in both directions 

 from the point of stimulation ; so that nerve-fibres which in 

 the intact body are afferent can conduct impulses towards 

 the periphery, and efferent fibres can conduct impulses away 

 from the periphery. In the normal state, however, double 

 conduction must seldom occur, for efferent fibres are con- 

 nected centrally, and afferent fibres peripherally, with the 

 structures in which their natural stimuli arise. In general, 

 too, an impulse, if it did pass centrifugally along an afferent 

 fibre, would not give any token of its existence, for the peri- 

 pheral organ would not be able to respond to it ; and we 

 have no reason to believe that the central mechanisms con- 

 nected with afferent fibres are better fitted to answer such 

 foreign and unaccustomed calls as impulses reaching them 

 along normally efferent nerves. There is some evidence that 

 muscular excitation is not carried over to the motor nerve- 

 fibres ; in other words, the wave of action flows from the 

 nerve to the muscle, but cannot be got to flow backwards. 

 Whether such an organ as the retina can be excited by 

 impulses reaching it ' the wrong way ' along the optic nerve 

 we do not know, although the point might possibly be 

 decided by means of the retinal currents to be mentioned 

 later on (p. 647). We shall see that a nutritive influence is 

 exerted over the afferent fibres of the spinal nerves by the 

 ganglion cells of the posterior root ganglia (p. 607), an influ- 

 ence which must spread along these fibres in the opposite 

 direction to that of the normal excitation ; but from this we 

 cannot deduce anything as to the behaviour of ordinary 

 nerve-impulses. 



The best proofs of double conduction in nerves, with 

 artificial stimulation, are : (i) The propagation of the nega- 

 tive variation or action current in both directions. This 

 holds for sensory as well as for motor fibres, as du Bois- 

 Reymond showed on the posterior roots, of the spinal nerves 

 of the frog and the optic nerves of fishes. (2) Stimulation 

 of the posterior free end of the electrical nerve of Malap- 



