THE SYMPATHETIC NERVE. 577 



over the whole of the vase-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 blood- 

 vessels, but, like other nerve-centres, may be the means by 

 which impulses are reflected (p. 486). And reflex actions 

 occur in connection with the muscular fibres of blood-ves- 

 sels, as with those of the voluntary muscles. Such reflected 

 impressions may lead either to contraction or to dilatation 

 of blood-vessels ; 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 blood-vessels, 

 is the depressor branch of the vagus (p. 563) ; but similar 

 effects have been observed in a less degree, on stimulating 

 other afferent spinal nerves.* 



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 blood-vessels, 

 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 processes of organic life, in 

 man, as liable to the combined influences of the cere- 

 bro-spinal and the sympathetic systems ; to consider that 

 those influences may be so combined as that the sympa- 



* For an admirable summary of what is at present known regarding 

 the Innervation of the Heart and Blood-vessels, see Lectures by Dr. 

 Uiitherford, in the "Lancet," December 16, 1871, and January 20, 

 1872. 



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