766 NERVE 



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 connected 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, wo.uld not give any token of its existence, for the 

 peripheral organ would not be able to respond to it ; and there is no 

 ground for assuming that the central mechanisms connected with 

 afferent fibres are better fitted to answer such foreign and un- 

 accustomed calls as impulses reaching them along normally efferent 

 nerves. There is good 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. Excitation of the central end of an efferent 

 (anterior) spinal root is not transferred to the corresponding afferent 

 (posterior) root, the connection between the efferent and afferent 

 neurons presenting the character of a physiological ' valve,' which 

 permits impulses to pass only in one direction. We have seen that 

 vaso-dilator impulses possibly pass out to the limbs over fibres 

 which, morphologically speaking, are afferent fibres (p. 179). And 

 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. 770), an influence which must spread along these fibres 

 in the opposite direction to that of the normal excitation. 



The best proofs of double conduction in nerves, with artificial stimu- 

 lation, are: (i) The propagation of the negative 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 Malapterurus (p. 813) causes 

 discharge of the electric organ, although the nerve-impulse travels nor- 

 mally in the opposite direction. (3) If the lower end of the frog's 

 sartorius is split into two, gentle stimulation of one of the tongues causes 

 contraction of individual fibres in the other. This is supposed to be due 

 to conduction of the nerve-impulse up a twig of a nerve-fibre distributed 

 to the one tongue, and down another twig of the same fibre going to the 

 other tongue. A similar experiment can be done on the gracilis of the 

 frog. This muscle is divided by a tendinous inscription into two parts, 

 each supplied by a branch of a nerve which divides after entering the 

 muscle. Stimulation of either twig is followed by contraction of both 

 parts of the muscle (Kuhne). 



Bert's much-quoted experiment on the rat is valueless as a proof of 

 double conduction. He caused union of the point of the tail with the 

 tissues of the back, then divided the tail at the root, and found that 

 stimulation of what was now the distal end caused pain. From this he 

 concluded that the sensory fibres of the ' transposed ' tail conducted 

 in the direction from root to tip. But the conclusion is not warranted, 

 for sensation disappeared in the tail after the section, and did not 



