NERVOUS ACTION. 391 



Nervous Acts. In some few instances they may be supposed 

 to originate sucli force, as exemplified in the persistent con- 

 traction in which are maintained the sphincter muscles of 

 the anus and neck of the bladder. Some nervous centres 

 (in the brain) are alone capable of taking knowledge of 

 sensory impressions ; and these impressions accordingly, if 

 from distant parts, and especially from such organs as are 

 supplied by the sympathetic nerve, have to traverse one or 

 more ganglia before they can be made patent to the mind. 



The nerve-fibres are incapable of generating nervous force 

 of themselves. Their sole functions are to convey such 

 force generated in the nervous centres to the peripheral 

 parts of the body, and to convey sensations and impressions 

 from such parts to the centres. They are simply conductors 

 of nervous force, and are aptly represented by the conduct- 

 ing wires of an electric machine, whilst the centre, as the 

 generator of force, is the analogue of the machine itself. 



The white fibres make up a great portion of the brain 

 and spinal cord, as well as almost the entire bulk of the 

 nervous trunks, with which these are directly connected. 

 They seem much more tough and resistent in the nerves 

 than in the centres ; but as the fibres are in the two cases 

 identical, this appearance is entirely dependent on the 

 amount of white fibrous tissue by which the nervous fibres 

 are enveloped. 



The grey fibres make up the main part of the various 

 branches of the sympathetic nerve, and enter in varying 

 proportion into the formation of the cerebro-spinal nerves. 



Nerve-fibres lie side by side in the nerves, but throughout 

 their whole course they maintain an entire independence of 

 each other. There is no union of individual fibres in the 

 nerve, nor any bifurcation of single fibres to permit a more 

 extensive distribution. A nerve accordingly possesses a 



