174 PHYSIOLOGY OF NERVE [CII. XV. 



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 more recently 

 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 

 cut sympathetic, in the course of some weeks the vagus fibres grow 

 into the sympathetic and form synapses around the cells of the 

 superior cervical ganglion, and stimulation of the united nerve now 

 produces such effects as are usually obtained when the cervical 

 sympathetic is irritated ; for instance, dilatation of the pupil, raising 

 of the upper eyelid, and constriction of blood-vessels of the head and 

 neck. (See accompanying diagram, fig. 192). 



Such experiments as these are important because they teach us 

 that though the action of nerves may be so different in different 

 cases (some being motor, some inhibitory, some secretory, some 

 sensory, etc.), after all what occurs in the nerve trunk itself is 

 always the same ; the difference of action is due to difference either 

 in the origin or distribution of the nerve-fibres. If we remember 

 the familiar illustration in which nerve trunks are compared to 

 telegraph wires, we may be helped in realising this. The destina- 

 tion of a certain group of telegraph wires may be altered, and the 



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

 (p. 198). 



