CH. XIV.] CKOSSING OF NKBVES 161 



organ, although the nervous impulse normally travels in the opposite 

 direction. 



Crossing of Nerves. 



Some experiments designed to prove the possibility of nervous 

 conduction in both directions were performed many years ago by 

 Paul Bert. He grafted the tip of a rat's tail either to the back of 

 the same rat, or to the nose of another. When union had been 

 effected, the tail was amputated near its base. After a time, irritation 

 of the end of the trunk -like appendage on the back or nose of the 

 rat gave rise to sensation. The impulse thus passed from base to 

 tip, instead of from tip to base, as formerly. This experiment does 

 not, however, prove the point at all ; for all the original nerve-fibres 

 in the tail must have degenerated, and the restoration of sensation 

 was due to new fibres, which had grown into the tail. Exactly the 

 same objection holds to another series of experiments, in which the 

 motor and sensory nerves of the tongue were divided and united 

 crosswise. Eestoration of both movement and sensation does occur, 

 but is owing to new nerve-fibres growing out from the central stumps 

 of the cut nerves. 



Though these experiments do not prove what they were intended 

 to, they are of considerable interest in themselves. Dr E. Kennedy 

 has recently carried out a very careful piece of work on this question 

 of nerve-crossing. He cut in a dog's thigh the nerves supplying 

 the flexor and the extensor muscles, and sutured them together 

 crosswise. Eegeneration of structure and restoration of function 

 occurred equally quickly, as in those cases in which the 

 central ends had been united to the peripheral ends of their own 

 proper nerves. On examining the cortex of the brain in those 

 animals in which nerve-crossing had been accomplished, it was 

 found that stimulation of the region which in a normal animal gave 

 flexion, now gave extension of the limb, and vice versd. 



A series of equally important experiments have also been carried 

 out by Langley, in which he shows that the same facts are true for 

 the nerves that supply involuntary muscle. These nerve-fibres 

 will under certain experimental conditions terminate by arborising 

 around other nerve-cells than those which they normally form 

 connections (synapses) * with. It will be sufficient to give one 

 typical experiment. If the vagus nerve is cut across in the neck, its 

 peripheral end degenerates downwards ; if the cervical sympathetic 

 is cut across below the superior cervical ganglion, its peripheral end 

 degenerates upwards, as far as the ganglion. If subsequently the 

 central end of the cut vagus is united to the peripheral end of the 



The meaning of the term " synapse " is fully explained in Chapter XVI. 



(p. 188). 



L 



