THE TISSUES 79 



movements of the dendrites or of the gemmules upon them 

 may occur. The activity of neurons is made evident 



(a) By their action upon other structures, e.g. muscles, 

 glands, etc., either (a) directly or (/3) indirectly through other 

 neurons. 



(b) By changes in the consciousness. 



(c) By electric changes in the neurons. 



(a) and (&) Action on other Structures. The activity of the 

 outgoing neurons neurons conducting impulses from the central 

 nervous system to muscles, glands, etc. is manifested by changes 

 in the muscles or other structures to which they go : while the 

 activity of ingoing neurons is made evident by their action on 

 outgoing neurons to muscles, etc., and sometimes by modi- 

 fications in the state of consciousness, which may be of the 

 nature of a simple brief sensation, or, by the implication of a 

 number of other neurons, may develop into a series of changes 

 accompanied by a corresponding series of sensations. 



Very interesting results follow from this fact that the 

 activity of neurons is made manifest by changes in the 

 structures to which they pass. Langley has demonstrated 

 that if the vagus, which conducts downwards to the abdominal 

 viscera, be cut, and the cervical sympathetic, which conducts 

 upwards to the head, be also cut, and the central end of the 

 vagus united to the peripheral end of the sympathetic, fibres 

 grow outwards from the vagus 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 ex- 

 tensors of a dog's forelimb be cut, and the central end of 

 the former united to the peripheral end of the latter, and 

 vice versd, co-ordinate movements occur, and that if that 

 part of the brain which naturally causes extension be stimu- 

 lated, 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. 



(c) Electrical Changes. The part of the neuron in action is 



