THE NEKVOUS SYSTEM 305 



entrance, and a smaller area below it. The finest twigs of these 

 branches are to be seen in the vicinity of the cell-bodies and 

 dendrites of certain other neurones. The axons of these second 

 links arborize in a similar way in the vicinity of large motor cells, 

 whose axons in turn become fibres of anterior roots. (For sim- 

 plicity's sake no reference is made to hosts of other neurones 

 which link the ganglion-cell and the motor-cell to other cells 

 higher in the cord or brain.) An impulse generated in the 

 sense-cell on the surface of the body runs up the root neurone 

 into the cord, where the ultimate twigs of the posterior root 

 fibre offer it a wide choice of distribution. Following the path 

 of least resistance, it passes into neurone No. 2. Again, the 

 arborization of No. 2 offers it alternative paths. It makes a 

 choice which lands it in No. 3. No. 3 passes the impulse on 

 to the muscle-fibres with which it is connected. Three 

 points are especially worthy of attention : (1) The impulse 

 has a wide (literally, an unlimited) choice of routes. The skin 

 of the finger is touched. Any muscle may respond, although 

 resistance is so graded as to cause the impulse to seek in the 

 first instance the group of muscles which is most often required 

 to act in consequence of stimulation of the finger. This 

 means, we may suppose, that it follows the chain which, 

 having the smallest number of links, offers least resistance. 

 If it cannot get through to these muscles, owing to the fact 

 that other impulses, acting simultaneously, either increase the 

 resistance in this particular path, blocking its way, or reduce 

 the resistance in an alternative path, it spreads farther afield. 

 (2) Owing to the ramification of the root-fibre which conveys 

 it to the cord, an impulse is not limited to a single line of dis- 

 tribution. It reaches many secondary links. It may therefore 

 influence various effector neurones simultaneously. For ex- 

 ample, a stimulus which calls extensor muscles into action, at 

 the same time inhibits their flexor antagonists. (3) The path 

 which it finally takes is accessible to all other impulses. Its 

 root neurone was peculiar to itself. Link No. 2 was more or 

 less a common path. Neurone No. 3 is open to every impulse 

 which traverses the nervous system. 



Anatomy justifies the construction of the scheme just out- 

 lined. But there are many points regarding structure upon 

 which a physiologist desires information, many details that he 



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