NERVOUS CONTROL OF THE BLOOD-VESSELS 995 



The two first details can be found by stimulating various nerves and 

 nerve-roots in different parts of their course and observing the effects pro- 

 duced on the local and general circulation. The importance of the third 

 heading is due to the fact that the vascular nerves, like the visceral nerves 

 generally, do not have their last cell-station in the spinal cord. The fibres 

 carrying vaso-constrictor impulses, on leaving the cord, do not pass direct 

 to the blood-vessels, but come to an end in a collection of ganglion-cells, 

 which may belong to the main chain of the sympathetic, or be situated more 

 peripherally and belong to the group of collateral or peripheral ganglia. 

 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 peripheral vascular nerves from the vaso- 

 motor centre. Stimulation of the pre-ganglionic fibre is now without effect, 

 although the normal results follow stimulation of the post-ganglionic non- 

 medullated fibre. 



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 

 Vieussenii 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. 



