CHAP, iv.] THE VASCULAR MECHANISM. 209 



muscular walls of the arteries are not mere passive instruments 

 worked by the cerebro-spinal axis through the cervical sympa- 

 thetic; obviously they have an intrinsic tone of their own, de- 

 pendent possibly on some local nervous mechanism, though in 

 the car at least no such mechanism has yet been found ; and it 

 seems natural to suppose that when the central nervous system 

 causes dilation or constriction of the vessels of the face, it makes 

 use, in so doing, of this intrinsic local tone. But if so, then the 

 simple view entertained above, that arterial dilation and constric- 

 tion are simply determined by the decrease or increase of tonic 

 constrictive impulses passing directly from the central nervous 

 system, is not a complete representation of the facts. 



In the second place, if we turn from the sympathetic or 

 splanchnic to other nerves containing vaso-motor fibres, we meet 

 with still greater difficulties. To take, for instance, a nerve sup- 

 plying a muscle, such as that going, in the frog, to the mylo-hyoid 

 muscle. Here, as in the cervical sympathetic, section of the nerve 

 produces dilation, but that dilation is even more transient than in 

 the case of the sympathetic; the vessels speedily return to their 

 former calibre. And then it is found that stimulation of whatever 

 strength of the peripheral portion of the divided nerve brings about 

 not constriction but dilation. A similar dilation is seen when the 

 nerve of a mammalian muscle is stimulated, and probably occurs in 

 the case of all muscular nerves. There are therefore in the body 

 nerves, stimulation of which, as well as mere section, always brings 

 about arterial dilation. 



There are other nerves in the body of a mixed character, 

 intermediate between the cervical sympathetic on the one hand, 

 and the muscular nerves on the other, stimulation producing now 

 constriction, now dilation. Such a nerve is the sciatic of a mam- 

 mal. We have already seen that section of this nerve produces 

 dilation of the vessels of the foot ; but the dilation so caused after 

 a few days disappears ; the foot on the side on which the nerve was 

 divided becomes not only as cool and pale, but frequently cooler 

 and paler than the foot on the sound side. If the peripheral por- 

 tion of the divided nerve be stimulated with an interrupted current, 

 immediately or very shortly after division, the dilation due to the 

 division gives place to constriction ; the sciatic acts then quite like 

 the cervical sympathetic, except perhaps that this artificial con- 

 striction cannot be maintained for so long a time, and is very apt 

 to be followed by increased dilation. If however the stimulation 

 be deferred for some days, until the dilation has given place to a 

 returning constriction, the effect is not constriction but dilation; 

 the nerve then acts, as far as its vaso-motor fibres are concerned, 

 like a muscular nerve and not like the cervical sympathetic. In 

 fact, by variations in the attendant circumstances, and in the mode 

 of stimulation, into the details of which we cannot enter now, 

 stimulation of the divided sciatic may at the will of the experi- 



p. H 



