THE NERVOUS SYSTEM 327 



The interval which elapses between the healing of the wound 

 in the wrist and the restoration of sensation and power of 

 movement is occupied in their downgrowth. 



The re-connection of regenerated nerves with their terminal 

 apparatus presents to the mind a curious problem. There is 

 no evidence that as function is re-established the brain has to 

 re-learn the situation of the sensory spots on the skin, or to 

 re-acquire skill in using the muscles which again come under 

 its control. From the moment that the outgrowing nerves 

 have recovered their terminal connections skin and muscles 

 have their right representation in the brain, however much the 

 two cut ends may have been twisted in their relation one to 

 another. It seems inconceivable that each nerve-fibre can 

 find its way to its original station ; but if it does not, our con- 

 ception of the mode of working of the nervous system still 

 needs much refining from the telephone-exchange analogy by 

 which we naturally help out our explanations. If a telephone 

 cable has been severed, it can be made useful again only in one 

 of two ways. Either the two segments of every wire that has 

 been cut must be reunited, or the subscribers' numbers must 

 be redistributed. 



The experiment of uniting the proximal segment of one nerve 

 with the distal segment of another of a quite different function 

 gives results which have an even more disconcerting effect upon 

 our theory of the nervous system. The sympathetic cord of the 

 neck and the vagus nerve lie very close together, alongside the 

 carotid artery. The vagus is both afferent and efferent. The 

 sympathetic is wholly efferent i.e., it conducts impulses, which 

 enter the sympathetic chain within the thorax, in the direction 

 of the head. If both nerves are cut, and the end of the vagus 

 turned round, so that it is in apposition with the upper end of 

 the sympathetic, its regenerating fibres make their way along 

 the sympathetic cord, headwards, to the superior cervical 

 ganglion. They arborize about the bodies of its ganglion- 

 cells, just as the sympathetic fibres used to do. The vagus 

 is a nerve of many functions. Amongst others, it inhibits 

 the contraction of the heart, constricts the bronchi of the 

 lungs, dilates the bloodvessels of the intestines, and helps in 

 regulating the movements of these viscera. After it has taken 

 the place of the upper segment of the sympathetic it dilates 



