236 THE CIRCULATION OP THE BLOOD 



arrive, then a hitherto dormant power of tonic activity becomes devel- 

 oped in the subsidiary centers. 



Independent Tonicity of Blood Vessels 



Even after complete disconnection of the spinal cord from the blood 

 vessels, as by cutting of the splanchnic nerve to the abdomen or abla- 

 tion of that portion of the lower spinal cord from which the fibers to 

 the hind limb arise, the disconnected blood vessels, although at first 

 completely dilated, may later reacquire an independent tone of their 

 own, indicating therefore, that they must possess some neuromuscular 

 mechanism which can act independently of the nerve centers, and which 

 may be stimulated to activity by the presence of hormones in the blood. 

 The hormone was at one time thought to be cpinephrine (see page 745). 



Epinephrine control is indicated in the effect produced upon arterial 

 blood pressure by stimulation of the great splanchnic nerve. Careful 

 analysis of the curve, shown in Fig. 29, shows that the rise is both im- 

 mediate and delayed; that is, the curve mounts immediately, then flat- 

 tens out a little, and then assumes a further rise. This delayed response 

 seems to depend upon the excretion of epinephrine into the blood, for it 

 does not occur when the suprarenal veins are occluded, and is much de- 

 layed by temporarily clamping the suprarenal veins on the same side 

 as that on which the splanchnic nerve is stimulated. It has been stated 

 by certain observers that, after occlusion of the adrenal veins, there is 

 a downward tendency of the blood pressure, which however develops 

 with extreme slowness; and that a distinct elevation of blood pressure 

 follows the removal of a clamp temporarily placed on the adrenal veins. 

 This rise is {^renounced if the splanchnic nerve is stimulated during the 

 occlusion of the veins. It must of course be understood that the imme- 

 diate rise in blood pressure following splanchnic stimulation is caused by 

 vasoconstriction in the splanchnic area itself, as is evidenced by the 

 fact that it does not occur, or is only very faint, when the abdominal 

 blood vessels are ligated prior to the stimulation of the splanchnic nerve. 

 Even after ligation of the adrenal veins and of the blood vessels of the 

 splanchnic area, stimulation of the splanchnic nerve may still cause a 

 slight rise in arterial blood pressure, possibly because some fibers may 

 run from the splanchnic to vascular areas not situated within the realm 

 of the splanchnic nerve for example, the blood vessels of the lumbar 

 muscles. 



