THE SYMPATHETIC NERYE. 573 



pressions is effected through the cerebro-spinal fibres 

 which are mingled in all, or nearly all, parts of the sym- 

 pathetic nerves. There are no means of deciding this ; but 

 if it be admitted that the conduction is effected through 

 the cerebro-spinal nerve-fibres, then, whether or not they 

 pass uninterruptedly between the brain or spinal cord and 

 the part affected, it must be assumed that their mode of 

 conduction is modified by the ganglia. For, if such 

 cerebro-spinal fibres are conducted in the ordinary manner, 

 the parts should be always sensible and liable to the in- 

 fluence of the will, and impressions should be conveyed to 

 and fro instantaneously. But this is not the case ; on the 

 contrary, through the branches of the sympathetic nerve 

 and its ganglia none but intense impressions, or impressions 

 exaggerated by the morbid excitability of the nerves or 

 ganglia, can be conveyed. 



Either, therefore, the nerve-fibres conduct differently in 

 the sympathetic nerves (which is improbable), or else the 

 ganglia have a power of modifying the method of conduc- 

 tion of impressions. It is as if the facility with which an 

 impression may be communicated from one fibre to another 

 in the ganglia were such, that the whole force of ordinary 

 impressions on the nerve-fibres is lost in diffusion among 

 the rest of their contents. This seems not improbable ; 

 for some cases show that when fibres certainly belonging 

 to cerebro-spinal nerves pass through ganglia of, or con- 

 nected with, the sympathetic, they do not so rapidly or so 

 surely transmit impressions as when they have no such 

 relation to the ganglia. Thus, the iris is not under the 

 direct or perfect influence of the will, though the passage 

 thereto of filaments of the third nerve, is shown by its 

 acting with the muscles supplied by the same nerve. 

 Neither does it always contract when the third nerve is 

 irritated, and when all the other muscles supplied by the 

 same nerve are put in action. So, also, when all the other 

 muscles supplied by the facial nerve contract on irritating 



