60 VETERINARY PHYSIOLOGY 



into the sympathetic, and when the vagus is stimulated, the 

 results which naturally follow stimulation of the sympathetic 

 occur. 



Kennedy has shown that, if the nerves to the flexors and 

 the nerves to the extensors of a dog's forelimb be cut, and 

 the central end of the former united to the peripheral end of 

 the latter, and vice versa, the normal co-ordinate movements 

 of the limb are restored, and that, if that part of the brain 

 which naturally causes extension be stimulated, flexion 

 occurs. He has applied the information thus gained to the 

 treatment of abnormal conditions in the human subject. In 

 a woman who suffered from spasmodic action of the muscles 

 of the face supplied by the seventh cranial nerve, he divided 

 this nerve and connected its peripheral end with the central 

 end of the spinal accessory and thus secured a complete 

 recovery. Such observations are of great interest, since they 

 indicate that the activity of the cells and synapses from 

 which the fibres come may undergo profound alteration in 

 function, that they may in fact be trained or educated to 

 take on activities not natural to them, 



(2) Electrical Changes. — The part of the neuron in action 

 is electro-positive to the rest of the neuron. This simply 

 means that the parts in action become to the rest of 

 the neuron what the zinc plate (the electro-positive 

 element) in a zinc and copper galvanic cell is to the copper. 

 The flow of current set up along the wire connecting the 

 elements of the cell is made manifest by the deflection of a 

 galvanometer needle. In the same way the zinc-like 

 (electro-positive) action of the acting part of the neuron is 

 made manifest if wires are led off from the nerve round 

 a galvanometer (fig. 22). 



In order that neurons may produce their effect they must 

 be capable of Excitation and Conduction, and these must 

 now be studied. 



2. Excitation of Neurons. 



Neurons, like all other protoplasm, react to changes in 

 external conditions ; they are capable of stimulation. 



