162 



PHYSIOLOGY OF NERVE 



[Cll. XiV. 



cub 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. 164.) 



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 



A B C 



Superior! 

 Cervical i 

 Ganglion 



.:: 



FIG. 164. Diagram to illustrate Langley's experiment on vagus and cervical sympathetic nerves. In 

 A, the two nerves are shown intact ; the direction of the impulses they normally carry is shown by 

 arrows, and the names of some of the parts they supply are mentioned. In B, both nerves are cut 

 through. The degenerated portions are represented by discontinuous lines. In C, the union 

 described in the text has been accomplished, and stimulation at the point a' now produces the same 

 results as were in the intact nerves (A) produced by stimulation at a. 



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 

 alteration may produce different consequences at different places ; 

 the electric change, however, in the wires would be the same in all 

 cases. So the nerve impulse going along a nerve is always the same 

 sort of molecular disturbance ; if it is made as in the experiment just 

 described, to go by a wrong channel, it produces just the same results as 

 though the impulse had reached its destination by the usual channel. 



