84 THE CIRCULATION OF THE BLOOD 



THE NERVOUS CONTROL OF THE BLOOD-VESSELS 



Vaso-Constrictor Nerves 



In 1852 Claude Bernard showed that in the rabbit 

 when the cervical sympathetic was cut, the arteries of the 

 ear dilated, and when the peripheral end of the nerve was 

 stimulated, the vessels contracted. He thus demon- 

 strated not only that the sympathetic conveyed vaso- 

 constrictor fibres, but that these exerted upon the vessels 

 a constant tonic action, removed by section of the nerve. 

 Vaso-constriction is now known to be a function of the 

 whole sympathetic system, and the origin of the impulses 

 has been traced to a centre — the " vaso-motor " centre, 

 situated in the floor of the fourth ventricle. From this 

 region impulses are constantly passing down the cord, which 

 they leave by the sympathetic outflow in the thoracico- 

 lumbar region. When the cord is transected at the seventh 

 cervical segment or higher, a maximal fall of blood-pressure 

 occurs, and any organ inserted in a plethysmograph under- 

 goes an increase in volume, owing to withdrawal of this 

 vaso-constrictor influence from all the blood-vessels in the 

 body which are provided with sympathetic fibres. When 

 the cord is transected at the third lumbar segment the 

 blood-pressure is unaffected, showing that no vaso-con- 

 strictor fibres issue from the cord below this level. The 

 blood-pressure is similarly unaffected when the brain-stem 

 is cut above the fourth ventricle, proving that the con- 

 trolling centre is below this level. But when the fourth 

 ventricle is itself destroyed complete fall of blood-pressure 

 results. A region in the fourth ventricle therefore presides 

 over the condition of the arterioles, and determines the 

 resistance which these vessels present to the outflow of 

 blood from the arterial reservoir. 



Details of the paths taken by vaso-constrictor fibres 

 are fully given in the section on the Autonomic System. 

 It is sufficient to state here that all these fibres emerge 



