NERVE 59 



1. Manifestations of the Activity of Neurons. 



When conducting, nerve, like a telegi'aph wire, manifests 

 no visible change. Its activity is shown chiefly by changes 

 in the structures to which it passes, but also by certain 

 electrical disturbances in the conducting part. 



(1) Action on other Structures. — The activity of the out- 

 going 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 (a) by 

 their action on outgoing neurons to muscles, etc. (see fig. 



A B 



Fig. 21. — To show the union of the vagus A to the anterior end of the sym- 

 pathetic B in the neck. The part of the central nervous S3'steni which 

 norniall3' acted upon the abdominal viscera becomes trained to act upon 

 the structures in the face and head. 



17, p. 5 3), and (6) 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 accomj^anied 

 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 

 structure to which they jxiss. 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 (fig. 21), fibres grow outwards from the vagus 



