274 COURSE OF VASO-CONSTRICTOR FIBRES. [BOOK i. 



anterior roots of spinal nerves, and then passing through the 

 appropriate visceral branches, join the thoracic or abdominal 

 sympathetic ganglia. In their course the fibres undergo a re- 

 markable change. 



Along the anterior root and along the visceral branch they are 

 medullated fibres, but before they reach the blood vessels for 

 which they are destined they become non-medullated fibres ; they 

 appear to lose their medulla in some or other of the ganglia. 



We are in many cases able to determine experimentally by the 

 following method, the ganglion or ganglia in which particular 

 fibres end, that is to say in which they become connected with 

 nerve cells. It is found that the drug nicotin abolishes or sus- 

 pends the action of vaso-motor fibres and of other fibres running 

 in the sympathetic system. Thus in a rabbit, after a certain dose 

 of nicotin has been given, stimulation of the cervical sympathetic 

 nerve in the neck no longer causes constriction of the vessels of 

 the ear. But it is found in such cases that though stimulation of 

 the trunk of the nerve in the neck is without effect, stimulation 

 of the appropriate nerve branches passing off from the superior 

 cervical ganglion on their way to the ear, does produce constric- 

 tion of the vessels of the ear. Obviously the nicotin does not 

 affect the peripheral fibres and endings of the nerve, but some 

 part of the nerve more central than the branches proceeding from 

 the superior cervical ganglion. Further, if the ganglion itself be 

 cautiously painted with a weak (1 p.c.) solution of nicotin, care 

 being taken to avoid excess, stimulation of the nerve in the neck 

 has no effect on the vessels of the ear, whereas if the nicotin be 

 applied to a corresponding extent to the trunk of the nerve in the 

 neck, none being allowed to have access to the ganglion, stimu- 

 lation of the trunk in the neck, even if applied to the very spot 

 on which the nicotin has been placed, produces the usual con- 

 striction of the vessels of the ear. Obviously the nicotin produces 

 its paralysing effects by acting on the nerve cells, or on the fibres 

 just as they are becoming connected with nerve cells. If the 

 solution of nicotin be applied not to the upper, but to the middle 

 or to the lower cervical ganglion, stimulation of the nerve between 

 the ganglion and the spinal cord produces the usual constrictor 

 effects. This shews that the constrictor fibres pass through the 

 lower and the middle ganglion as fibres, not connected with cells, 

 otherwise they would be here affected by nicotin ; they are affected 

 by nicotin in the upper ganglion, and we therefore infer that they 

 end in, that is, are connected with cells in that ganglion. In the 

 same way it may be found that the vaso-constrictor fibres of the 

 abdominal splanchnic are connected with cells in the solar plexus. 

 Indeed by this method we may determine in what ganglia the 

 vaso-constrictor and other fibres of the sympathetic system end ; 

 and a remarkable distribution, determined by morphological 

 causes among others, has in this way been made out, some fibres 



