1038 PHYSIOLOGY 



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These fibres, as they leave the central nervous system, are small medullated 

 nerves. They end in the ganglion by arborising round ganglion cells, 

 whence a fresh relay of fibres starts and carries the impulses on to the 

 muscle fibres of the blood vessels. The post-ganglionic fibres differ from 

 the pre-ganglionic fibres in being non-medullated. 



The discovery of the ganglia, with which any given set of nerve fibres 

 is connected, is rendered easy by the fact that in many animals the sympa- 

 thetic ganglion cells are paralysed by nicotine (Langley). The nicotine 

 may be painted on the ganglion or may be injected into the blood stream. 

 The first effect of the drug is a powerful stimulation of the ganglion cells, 

 so that, if the drug be injected, there is an enormous rise of blood pressure 

 owing to the universal vaso-constriction that is produced. The stimulation 

 gives place to a condition of paralysis; the blood pressure falls below 

 normal, owing to the cutting off of the peripteral vascular nerves from the 

 vaso-motor centre. Stimulation of the pre-ganglionic fibres is now without 

 effect, although the normal results follow stimulation of the post-ganglionic 

 non-medullated fibres. 



By these methods it has been determined that all the vaso-constrictor 

 nerves of the body leave the spinal cord by the anterior roots of the spinal 

 nerves from the first dorsal to the third or fourth lumbar inclusive. From 

 the roots they pass by the white rami communicantes to the ganglia of the 

 sympathetic chain lying along the front of the vertebral column. Here 

 they take different courses according to their destination. 



The fibres to the head and neck leave by the first four thoracic nerves, 

 pass into the sympathetic chain through the ganglion stellatum and ansa 

 Vieu^senii to the inferior cervical ganglion, and up the cervical sympathetic 

 trunk to the superior cervical ganglion. Here they end, and the impulses 

 are carried by a fresh relay of fibres, which start from cells in this ganglion 

 and travel as non-medullated fibres on the walls of the carotid artery and 

 its branches. 



The constrictors to the fore limb in the dog leave the cord by the white 

 rami of the fourth to the tenth thoracic nerves. The fibres run up the 

 sympathetic chain to the stellate ganglion, where they all end in synapses 

 round the cells of this ganglion. The impulses are carried on by non- 

 medullated fibres along the grey rami of the sympathetic to the cervical 

 nerves which make up the brachial plexus, and run down in the branches of 

 this plexus to be distributed to the vessels of the fore limb. 



The constrictor impulses to the hind limb in the dog arise from the 

 nerve roots between the eleventh dorsal and third lumbar roots. All 

 the fibres end in connection with cells in the sixth and seventh lumbar 

 and first and second sacral ganglia of the sympathetic chain, whence 

 the impulses are carried by grey rami to the nerves making up the sacral 

 plexus. 



The most important vaso-motor nerve of the body is the splanchnic 

 nerve. This nerve receives most of the fibres forming the white rami from 

 the lower seven dorsal and upper two or three lumbar roots, the latter fibres 



