CHAP, iv.] THE VASCULAR MECHANISM. 337 



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 

 constriction 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 constriction 

 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- constrict or 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 very 

 speedily becoming connected with nerve cells, others running a 

 very long course before they so end. 



We may add that in the anterior roots, and along the visceral 

 branches, in fact until they become connected with cells these 

 fibres are invariably medullated fibres of small diameter, not more 

 than 1/8 //, to 3'6 p in diameter. 



170. Vaso-dilator Fibres. Some of these appear to run 

 much the same course as the vaso- constrictors. Such are the 

 vaso-dilator fibres running in spinal nerves like the sciatic and 

 brachial, those which seem to be present in the splanchnic, and 

 certain fibres of the cervical sympathetic which in some animals 

 at least act as vaso-dilators towards certain parts of the mouth 

 and face. With regard to these, the evidence of whose existence, 

 as we have seen, is at least in most cases, difficult, special or 

 indirect, we have at present no proof that their general course 

 is essentially different from that of the constrictors. 



F. 22 



