418 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



but also by an actual stagnation of the blood stream which results in consequence 

 of an obstruction to the venous return. The latter effect is made possible by the 

 contraction of circular cushions of muscular tissue which form sphincters at the 

 points of junction between the venules and the cavernous blood spaces. 



(b) The dilator and constrictor reactions may be dissociated by chemical means. 

 Thus, it has been found by Dale ^ that ergotoxin possesses the property of paralyzing 

 the constrictor mechanism, so that the stimulation of any mixed vasomotor nerve 

 must give way in time to dilatation. It is also possible to produce vasomotor 

 effects solely with the aid of chemical agents so that we need not resort to the 

 electrical stimulation of a nerve. For example, adrenalin in very small doses 

 dilates the blood-vessels of the cutaneous circuits, while larger doses give rise 

 to a constriction. ^ In the same way, it has been found that proteoses cause a 

 dilatation, while chrysotoxin (ergot) stimulates the constrictor mechanism, 



(c) The constrictor and dilator reactions may also be dissociated by changing 

 the temperature or by changing the frequency of the stimulation. Thus, Howell, 

 Budget and Leonard ^ have shown that the irritability of the dilator fibers of the 

 sciatic nerve may be destroyed sooner than that of the constrictors by simply 

 heating or cooling the nerve. If a quickly interrupted current of moderate strength 

 is applied to a nerve, the usual result is vasoconstriction. Bowditch and Warren,* 

 however, have found that infrequent electrical stimuli commonly give rise to a 

 dilatation instead of a constriction. In the case of the renal blood-vessels, 

 Bradford^ employed fifty induction shocks at intervals of one second. Very 

 similar results have been obtained with the greater splanchnic nerve, by Meltzer 

 and Auer,^ and Burton-Opitz.^ The infrequent excitation of the central end of this 

 nerve gave rise to reflex vasodilatation and a most pronounced fall in blood pressure. 

 It might also be mentioned that the degeneration following the division of the 

 sciatic nerve, affects the constrictor fibers first of all, so that vasodilator effects 

 may be obtained for some time after its constrictor power has been lost. 



The Results of the Reaction. — In general, it holds true that the 

 division of a nerve containing vasomotor fibers is followed by a 

 relaxation of the blood-vessels innervated by it. The vascular area 

 so affected loses its tonic resistance and becomes engorged with blood 

 and distinctly warm to the touch. If this area is sufficiently large, these 

 changes must, of course, react upon the general circulation and produce 

 a fall in the general pressure, because a considerable quantity of the 

 systemic blood must find its way into these relaxed vessels. In many 

 cases these blood-vessels regain their tonus within a comparatively 

 brief period of time, provided, of course, that they are still in connec- 

 tion with ganglionic elements. The latter are capable of assuming the 

 function of those chief centers with which they were previously con- 

 nected. This is especially true of the blood-vessels situated in the 

 realm of the sympathetic system, because this system embraces 

 numerous local conglomerations of ganglion cells which are markedly 

 independent in their function from the cerebrospinal structures. 



Most generally, the excitation of the distal end of a divided vaso- 



1 Jour, of Physiol., xlvi, 1913, 291. 



^ Hartman, Am. Jour, of Physiol., xxxviii, 1915, 438. 



3 Jour, of Physiol., xvi, 1894, 298. 



^ Ibid., vii, 1886, 416. 



6 Ibid., x, 1889, 358. 



6 Centralb. fur Physiol., 1916. 



' Am. Jour, of Physiol., xlii, 1917, 498. 



