572 THE NERVOUS SYSTEM. 



or spinal nerves differ always from those of the sympathetic 

 system, it is impossible in the present state of our know- 

 ledge to be sure of the source of fibres which from their 

 structure might lead the observer to believe that they arose 

 from the brain or spinal cord on the one hand, or from, 

 the sympathetic ganglia on the other. In other words, 

 although the large white tubular fibres are especially cha- 

 racteristic of cerebro-spinal nerves, and the pale or 

 gelatinous fibres of a sympathetic nerve, in which the}^ 

 largely preponderate, there is no certainty to be obtained 

 in a doubtful case, of whether the nerve-fibre is derived 

 from one or the other, from mere examination of its 

 structure. It may be derived from either source. 



The physiology of the sympathetic nerve is still very 

 obscure ; there are, however, certain statements which may 

 be made in regard to it. 



And first, it may be stated generally as nearly certain 

 that the sympathetic nerve-fibres are simple conductors of 

 impressions, as those of the cerebro-spinal system are, and 

 that the ganglionic centres have (each in its appropriate 

 sphere) the like powers both of conducting and of commu- 

 nicating impressions. Their power of conducting impres- 

 sions is sufficiently proved in ordinary diseases, as when 

 any of the viscera, usually unfelt, give rise to sensations 

 of pain, or when a part not commonly subject to mental 

 influence is excited or retarded in its actions by the various 

 conditions of the mind ; for in all these cases impressions 

 must be conducted to and fro through the whole distance 

 between the part and the spinal cord and brain. So, also, 

 in experiments, now more than sufficiently numerous, 

 irritations of the semilunar ganglia, the splanchnic nerves, 

 the thoracic, hepatic, and other ganglia and nerves, have 

 elicited expressions of pain, and have excited movements 

 in the muscular organs supplied from the irritated part. 



In the case of pain excited, or movements affected by 

 the mind, it may be supposed that the conduction of im- 



