CHAPTER XXVII 

 THE VASO-MOTOR NERVES 



THE bloodvessels are supplied with constrictor and dilator nerve 

 fibres which regulate the size of the vascular bed and the distribution 

 of the blood to the various organs. The arteries may be compared 

 to a high -pressure main supplying a town. By means of the vaso- 

 motor nerves the arterioles (the house taps) can be opened or closed, 

 and the current turned on to or off any organ according to its func- 

 tional needs. If all the arterioles be dilated at one and the same 

 time, the aortic pressure falls, and the blood, taking the pathways 

 of least resistance, gravitates to the most dependent parts of the 

 vascular system; just as, if all the taps in the town were opened at 

 once, the pressure in the main would fail, and only the taps in the 

 lower parts of the town would receive a supply. The discovery of 

 the vaso-motor nerves is due to Claude Bernard (1851). He dis- 

 covered that by section of the cervical sympathetic nerve he could 

 make the ear of a rabbit flush, while by stimulation of this nerve he 

 could make it blanch. He almost made the further discovery that 

 stimulation of certain nerves, such as the chorda tympani supplying 

 the salivary gland, produces an active dilatation of the bloodvessels. 

 The vaso-constrictor fibres issue in the anterior spinal roots (the white 

 rami), from the second thoracic to the second lumbar root, and pass 

 to the sympathetic chain of ganglia. The fibres are of small diameter, 

 and probably arise from cells situated in the lateral horn of th^-grey 

 matter of the spinal cord. They each have a cell station in one or 

 other ganglion, and proceed as post-ganglionic fibres to the cervical 

 sympathetic, to the mesenteric nerves, and back as the grey rami to 

 join the nerves of the limbs (Fig. 457). 



Nicotine paralyzes ganglion cell synapses, and by applying this test 

 to the various ganglia the cell stations of the vaso-constrictor fibres 

 supplying each organ have been mapped out. The vaso-dilator fibres 

 have not so restricted an origin, for they issue in the efferent roots in 

 all parts of the neural axis. The two kinds of nerves, although antag- 

 onistic in action, end in the same terminal plexus which surrounds the 

 vessels. The presence of vaso-dilator fibres in the common nerve trunks 

 is masked, on excitation, by the overpowering action of the vaso-con- 

 strictor nerves. The latter are, however, more rapidly fatigued than 

 the former, and by this and other means the presence of vaso-dilator 

 fibres can be demonstrated in almost all parts of the body. The 

 nervi erigentes to the penis and the chorda tympani supplying the 

 salivary glands are the most striking examples of vaso-dilator nerves. 



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