200 THE AUTONOMIC NERVOUS SYSTEM [CH. XVII. 



Furthermore, there are many fibres of the white rami which enter 

 the lateral chain of ganglia and pass through them without com- 

 municating with their cells at all, and never return to the spinal 

 nerves by grey rami. They pass out of the lateral chain either to 

 collateral or even terminal ganglia before reaching their cell-stations, 

 whence they emerge as post-ganglionic fibres. This is the case for 

 the sympathetic supply of the blood-vessels of and involuntary 

 muscular fibres of the thoracic, abdominal, and pelvic viscera, and is 

 therefore true for such important nerves as the cardiac accelerators, 

 the splanchnics, and the nervi erigentes. 



Fig. 189 shows the course of the splanchnic fibres, and will 

 assist the student in grasping this method of distribution. 



The great majority are arranged as in A, that is to say they have 

 their cell-stations in the solar ganglion. Comparatively few are 

 arranged as in B, where some fibres do not reach their cell-stations 

 until they arrive at the terminal ganglion situated in the walls of the 

 viscus (for instance, the pancreas) to which they are distributed. A 

 few possibly and occasionally are arranged as in C, with a cell -station 

 for some of their branches in the lateral sympathetic chain. 



It will be noticed that if any post-ganglionic fibre is traced back- 

 wards, there is one and only one cell-station between the central 

 nervous system and the ultimate distribution of the nerve fibrils. 



The next question that arises is, how have all these facts been 

 ascertained ; for it is obviously impossible to follow the individual 

 fibres with the microscope, and still less with the naked eye. The 

 method above all others which has proved successful in solving the 

 problem is the nicotine method, originally introduced by Langley 

 and Dickinson, and employed since by Langley mainly in conjunc- 

 tion with H. K. Anderson. 



The Nicotine Method. Nicotine in small doses paralyses nerve- 

 cells, but not nerve-fibres. Before the paralytic effect of nicotine 

 comes on, it excites the nerve-cells, and this in the case of the blood- 

 vessels causes a general constriction of the arterioles, and a con- 

 sequent rise of arterial pressure. It is still a matter of uncertainty 

 whether the drug produces these effects on the nerve-cells themselves 

 or on the terminal arborisations (synapses) of the fibres that sur- 

 round them, or on receptive substances (see p. 164) either in the 

 cells or present at the synaptic junctions. But whichever of these 

 modes of action is the correct one, the main result is the same; a 

 nervous impulse which reaches a ganglion by a pre-ganglionic fibre 

 cannot get across to the corresponding post-ganglionic fibres if the 

 animal is poisoned with nicotine. Stimulation of the anterior nerve- 

 roots, or of the white rami no longer produces movements of the 

 involuntary muscular tissues, because the paralysed cell-stations act 

 as blocks to the propagation of the impulses. If, however, post- 



