THE TISSUES 81 



3. Manifestations of the Activity of Neurons. So far as 



is at present known, the activity of neurons is not accom- 

 panied by any obvious change, although it is possible that 

 movements of the dendrites or of the gemmules upon them 

 may occur. The activity of neurons is made evident 



1st By their action upon other structures, e.g. muscles, 

 glands, &c., either (a) directly or (6) indirectly, through other 

 neurons. 



2nd. By electric changes in the neuron ; and 



3rd. By changes in the consciousness. 



The activity of the outgoing neurons neurons conducting 

 impulses from the central nervous system to muscles, glands, 

 &c. 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, &c., and sometimes by modifications 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 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. Kennedy has shown that, if 

 the nerves to the flexors and the nerves to the extensors 

 of a dog's thigh be cut, and the central end of the first 

 united to the peripheral end of the second, and vice versa, 

 co-ordinate movements occur, and that if that part of the 

 brain which naturally causes extension be stimulated, flexion 

 occurs. Langley has demonstrated that if the vagus which 

 conducts downwards to the abdominal viscera be cut, and 

 the cervical sympathetic which conducts up to the head be 

 also cut, and the central end of the vagus united to the 

 peripheral end of the sympathetic, vagus fibres grow out- 

 wards, and when the vagus is stimulated, the results which 

 naturally follow stimulation of the sympathetic occur. 



4. Conduction. It is found that if a neuron is stimulated 

 at any one point, some time elapses before the result of the 

 stimulation is made manifest, and that the farther the point 

 stimulated is from the structure acted upon, the longer will 

 be this latent period. This of course indicates that the 



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