THE NERVOUS SYSTEM 143 



Incoming Fibres. These take origin either in definite peri- 

 pheral structures, such as Pacinian corpuscles, or in some 

 less defined endings, and as medullated fibres pass throu^n 

 the various ganglia, and, so far as is at present known, have 

 their cell stations in the ganglion on the posterior root. 

 2nd, The Outgoing Fibres, characterised by their small size, 

 take origin chiefly in a lateral column of cells, which is well 

 developed in the dorsal region of the cord, and pass out as 

 medullated fibres by the anterior root. From this they pass 

 by the white root to a sympathetic ganglion, whence they 

 may proceed in one of two different ways. 



(a) They may form synapses with cells, and from these 

 cells fibres may pass 



1. Outwards with the splanchnic nerves ; or, 



2. Back into the spinal nerve by the grey root and so 

 down the somatic nerve to blood-vessels, hairs, sweat glands, 

 &c. The ganglia from which fibres pass back into spinal 

 nerve are known as lateral ganglia. 



(b) They may pass through these ganglia on to one more 

 peripherally situated in which they form synapses and are 

 continued onwards. These ganglia from which fibres do not 

 pass back are called collateral ganglia. Before their first 

 interruption they are termed pre-ganglionic fibres, after their 

 interruption post-ganglionic. 



The various fibres after their interruption proceed as non- 

 medullated or grey fibres to their termination, where they 

 break up into a network of anastomosing fibres with cells a 

 sort of terminal ganglion. Many drugs have a special action 

 on the terminal ganglia, e.g. apocodein paralyses them, while 

 adrenalin the extract of the medullary part of the supra- 

 renals stimulates them. 



The interruption of fibres in ganglia, or their passage 

 through these structures, has been determined by taking 

 advantage of the fact that nicotine in one per cent, solution 

 when painted on a ganglion poisons the synapses but does 

 not influence the fibre. Hence, when a ganglion is painted 

 with nicotine, if stimulation of the fibres on its proximal side 

 produces an effect, it is proved that the break is not in that 

 ganglion. 



