THE CIRCULATION 



303 



FIG. 163 



cells of the lateral tracts. From them small medullated fibers pass 

 outward as parts of the anterior roots and enter ganglia in the so-called 

 thoracic sympathetic chain and perhaps elsewhere in the body. From 

 these ganglia, probably of the nature of locally distributing centers, 

 unmedullated preganglionic fibers convey the impulses to the smooth 

 muscle-cells in the arteries, complex motor end-organs intervening 

 between the fibers and the muscle-units. 



The "vaso-dilator centers" and nerves have been less well located than 

 the preceding, their functional antagonists. Impulses causing vaso- 

 dilatation through a sort of inhibitory action seem to arise from various 

 regions of the cord all the way from the medulla to the sacrum. As is 

 the case with the vaso-constrictors, most of the dilator nerves come from 

 the thoracic segments of the cord and from the first and second lumbar 

 segments. No general vaso-dilator center is known. Coming from 

 these regions the fibers pass outward to 

 very many ganglia scattered in different 

 parts of the body, especially of the trunk. 

 Some of these ganglia are large and some 

 are small; some control single organs and 

 others considerable regions of the tissues 

 generally. These fibers bearing vaso- 

 motor impulses between the cord and 

 their destination are not usually separate 

 nerves. On the other hand, they generally 

 form parts of the complex bundles of 

 fibers making up "the nerves" of the body, 

 for these are mostly bearers of very many 

 different sorts of influences and messages. 

 The sciatic nerve, for example, contains 

 both kinds of vaso-motor fibers, and the 

 vaso-motor effects of the artificial stimula- 

 tion of the cut sciatic depends on the ex- 

 perimental conditions. The constrictor 

 influence usually at first overpowers the dilator, but soon becomes 

 fatigued in some way, leaving the dilator effect in control of the parts 

 supplied. The dilators, however, as Bowditch and Warren showed, 

 are more susceptible to weak and relatively infrequent stimuli than are 

 the constrictor centers. Whether the constrictor and the dilator influ- 

 ences act continually, making the tonus of the arteries thus a balance 

 of opposed forces, or whether (as is more likely) the constrictor centers 

 by themselves control the tone, the dilatation being passive, is as yet un- 

 certain. The dilator-influence on the latter supposition would be exerted 

 only occasionally, when, for example, rapid or marked dilatation was 

 necessary or when perhaps the blood-pressure was locally too low to 

 promptly open the arteries. Local actions are probably elaborately 

 provided for as regards both blood-pressure and vaso-motion. Indeed, 

 so distinct is this local control that it might almost be said that normally 



Tracing to show the fall of press- 

 ure in the carotid caused by vaso- 

 dilatation. (Meyer.) 



