688 A MANUAL OF PHYSIOLOGY 



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. Ex- 

 citation 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. 165). 

 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. 693), 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 

 stimulation, 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. 737) causes discharge of the electric organ, 

 although the nerve-impulse travels normally in the opposite direc- 

 tion. (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 con- 

 duction 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 return till some months later, when the nerve- 

 fibres, after degenerating, would have been replaced by new sensory 

 fibres growing down from the dorsal nerves (Ranvier) . For a similar 

 reason the so-called union of the peripheral end of the cut hypo- 

 glossal nerve (motor) with the central end of the cut lingual (sensory) 

 proves nothing as to double conduction, nor as to the possibility of 

 motor nerves taking on a sensory function. For while sensation is 

 after a time restored in the affected portion of the tongue, this is 

 due to the growth of sensory fibres from the central stump of the 

 lingual down through the degenerated hypoglossal, and not to the 

 conduction upwards of sensory impulses by the motor fibres of the 

 latter. 



