INNERVATION OF THE BLOOD-VESSELS OF DIFFERENT ORGANS 421 



SPECIAL VASOMOTOR REACTIONS 



The Spinal Cord. — As the spinal cord is the chief highway by- 

 means of which the vasomotor center in the medulla stands in com- 

 munication with the constrictor and dilator mechanisms of the blood- 

 vessels, the destruction of this part must lead to a pronounced fall 

 in blood pressure. The tonic influences of the higher center are then 

 prevented from reaching the periphery, as are also those generated in 

 the minor centers of the cord itself. In other words, a general vascular 

 relaxation now results which may finally produce an almost complete 

 stoppage of the blood flow. The animal, so to speak, is bled into its 

 own highly relaxed vessels. 



A fall in blood pressure may also be produced by dividing the cord 

 either in its cervical or in its thoracic region. In both cases the 

 blood-vessels innervated by those nervous elements which are situated 

 posteriorly to the cut, lose their tonus and relax. It is to be noted, 

 however, that this relaxation is not permanent, because the lower 

 spinal centers then develop a tonic power independent of that of the 

 rest of the cord. The blood-vessels gradually regain their former 

 caliber and enable the blood pressure to return to a value approaching 

 normal. From the foregoing data, it may also be inferred that the 

 excitation of the peripheral stump of the spinal cord must give rise to a 

 vasoconstriction and a rise in the general blood pressure, because the 

 constriction of the formerly relaxed blood-vessels leads to the trans- 

 fer of a large amount of previously stagnated blood into the general 

 circulatory system. The stimulation of the central stump of the 

 divided spinal cord with currents of ordinary strength sets up different 

 reflexes which usually result in a pressor reaction. 



The Sciatic Nerve. — This nerve must be considered as the vaso- 

 motor highway of the posterior extremity. In accordance with the 

 preceding analysis, it may be gathered that its division occasions a 

 relaxation of the blood-vessels innervated by it, but a marked reduction 

 in the general blood pressure cannot result in consequence of this 

 procedure, because the extra quantity of blood which finds its way into 

 the circulatory channels of the leg, is not sufficiently large to affect 

 the dynamic conditions in the general circulation. The stimulation 

 of the distal stump of the divided sciatic nerve is usually followed by a 

 constriction of the peripheral blood-vessels, the superfluous amount of 

 blood being again driven into the general circuits of the body. But 

 this transfer remains as a rule without decisive effect upon the general 

 circulation for the reason just given. The result ordinarily obtained 

 upon excitation of its central end is a rise in blood pressure, but this 

 pressor effect may be changed into a depressor reaction by lessening the 

 frequency and intensity of the stimuH. The foregoing account is also 

 applicable to other spinal nerves, such as the brachial. 



Our knowledge regarding the vasomotors of skeletal muscle tissue 

 is still very indefinite, owing to the difficulties experienced in differ- 



