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 sjonpathetic nerve, in which they 

 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 struc- 

 ture. It may be derived from either source. 



With respect to the functions of the sympathetic nervous 

 system, it may be stated generally 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 communicating impres- 

 sions. Their power of conducting impressions is suffi- 

 ciently 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, irrita- 

 tions 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- 

 pressions is effected through the cerebro-spinal fibres 

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



