CH. XXI.] 



VASO-MOTOR NERVES. 



303 



of the bladder, the efferent path to which is the other hypogastric nerve. 

 In addition they observed an apparent reflex excitation of the nerve sup- 

 plying the erector muscles of the hairs (pilo-motor nerves) through other 

 sympathetic ganglia. In neither case is the action truly reflex, but is 

 caused by the stimulation of the central ends of motor-fibres which issue 

 from the spinal cord, and which after passing through the ganglion send 

 branches down each hypogastric nerve. The experiment is in fact similar 

 to Kiihne's gracilis experiment (p. 173). 



A series of most interesting and important experiments have 

 more recently been carried out by Langley, in which he shows 

 that nerve-fibres will under certain experimental conditions 



A B C 



Superior 

 Cervical 

 Ganglion 



g 



1 



Fig. 285. 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 cam' 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 " ' now produces the same results us were in 

 the intact nerves (A) produced by stimulation at a. 



terminate by arborising around other nerve cells than those 

 which they normally form connections (synapses) with. It will 

 be sufficient to give one typical experiment. If the vagus nerve 

 is cut across in the neck, its peripheral end degenerates down- 

 wards ; if the cervical sympathetic is cut across below the 

 superior cervical ganglion, its peripheral end degenerates upwards, 

 as far as the ganglion. If subsequently the central end of the 

 cut vagus is united to the peripheral end of the cut sympa- 

 thetic, in the course of some weeks the vagus fibres -grow 

 into the sympathetic and form synapses around the cells of tho 

 superior cervical ganglion, and stimulation of the united nerve 



