BLOOD VESSELS 



459 



roots of the various spinal nerves, and do not pass through 

 the sympathetic gangha, but run as medullated fibres to 

 their terminal ganglia (fig. 191). Bayliss has shown that the 

 vaso-dilator fibres for the hind limb of the dog leave the cord 

 by the posterior roots. Evidence has been adduced that 

 dilatation can be brought about by irritation of the skin even 

 after the posterior root is cut above the ganglion, but not 

 when it is cut below it (p. 98). The possible action of this 

 secondary vaso-dilator mechanism in the production of inflam- 

 mation and of such trophic 

 disturbances as shingles or 

 herpes zoster is worthy of 

 attention. 



The probable existence of 

 Si peripheral vaso-dilator mech- 

 anism indicated by the action 

 of nitrites and of weak acids 

 may be of importance in ex- 

 plaining local dilatation of 

 vessels during the functional 

 activity of a part (p. 455). 



•2. Mode of Action. — 

 (a) Reflex Stimulation. — This 

 mechanism is not constantly 

 in action, since section of a 

 vaso-dilator nerve does not 

 cause constriction. It may, however, be excited reflexly. 



Stimulation of cm afferent nerve causes a dilatation of 

 the arterioles in the p)art from which it comes, and a con- 

 striction of the arterioles throughout the rest of the body. If 

 a sapid substance such as pepper be put in the mouth, the 

 buccal mucous membrane and the salivary glands become 

 engorged, while there is a constriction of the arterioles 

 throughout the body. The vaso-dilator mechanism is not 

 general in its action like the vaso-constrictor, but is specially 

 related to the different parts of the body. 



Again, it has been showm that stimulation of the central 

 end of the depressor nerve (superior cardiac branch of the 

 vagus) causes a dilatation of the arterioles, chiefly in the 



Fig. 192.— To show Local Vaso- 

 dilatation with General Vaso- 

 constriction. S., skin; V.D., 

 local vaso-dilator centre; V.C, 

 vaso-constrictor centre. 



