THE SYMPATHETIC NERVE. 453 



fibres which arise in the ganglia through which they pass. 

 The vaso-motor centre in the medulla appears to have a regu- 

 lating power over the whole of the vaso-motor nerves ; but it 

 seems likely that other secondary vaso-motor centres may 

 exist in ganglia in different parts of the body, and may be the 

 centres by which, under ordinary circumstances, vaso-motor 

 changes are regulated in the territory in which they are placed. 



The vaso-motor nerve-centres are not only centres from which 

 influences are directly transmitted to the bloodvessels, but, like 

 other nerve-centres, may be the means by which impulses are 

 reflected (p. 385). And reflex actions occur in connection with 

 the muscular fibres of bloodvessels, as with those of the vol- 

 untary muscles. Such reflected impressions may lead either 

 to contraction or to dilatation of bloodvessels ; or, in other 

 words, the action may be excito-vaso-motor, or vaso-inhibitory. 

 The most remarkable instance at present known of a nerve, 

 the stimulation of which leads by reflex action through the 

 vaso-motor centre in the medulla oblongata, to dilatation of 

 bloodvessels, is the depressor branch of the vagus (p. 442) ; 

 but similar effects have been observed in a less degree, on stimu- 

 lating other afferent spinal nerves. 1 



It is, of course, very difficult to determine the relative share 

 exercised by the true sympathetic and the ordinary cerebro- 

 spinal fibres in the contraction of bloodvessels, and in the 

 general processes of nutrition and secretion, since both kinds 

 of fibres appear to be distributed to most parts, and there seems 

 to be no possibility of isolating them. Probably the safest 

 view of the question at present is, still to regard all the pro- 

 cesses of organic life, in man, as liable to the combined influ- 

 ences of the cerebro-spinal and the sympathetic systems ; to 

 consider that those influences may be so combined as that the 

 sympathetic nerves and ganglia may be in man, as in the lower 

 animals, the parts through which the ordinary and constant 

 influence of nervous force is exercised on the organic processes ; 

 while the cerebro-spinal nervous centres and their ganglia are 

 so closely connected with the proper sympathetic ganglia, that 

 neither of them can be said to be independent of the other ; 

 each, as a rule, and under ordinary circumstances, governing 

 its own domain, but always liable to be influenced by the other. 

 _, 



1 For an admirable summary of what is at present known regard- 

 ing the Innervation of the Heart and Bloodvessels, see Lectures by Dr. 

 Eutherford, in the " Lancet," December 16th, 1871, and January 20th, 

 1872. 



