88 



THE CIRCULATION OF THE BLOOD 



far the stronger. When a vaso-constrictor nerve is cut, 

 impulses which were previously passing down it are 

 aboUshed, and the arterioles which it supphes are dilated. 

 But when a vaso-dilator nerve is cut there is hardly 

 any constriction. The vaso-dilators, then, under normal 

 conditions exert but a feeble, if any, effect. 



Does the wall of the arteriole possess an inherent 

 tonus independent of any nervous influence ? It would 



Fig. 20. 

 B = Posterior Root Fibre, the axon dividing distally, one part 



supplying the skin, the other a blood-vessel A which it dilates. 

 C = Motor Fibre to muscle M. 

 D = Sympathetic pre-ganglionic fibre. 

 E = Post-ganglionic fibre arising in a sympathetic ganglia. Distally 



it supplies the blood-vessel with vaso-constrictor fibres and 



innervates the hairs and sweat glands. 



seem that it does, because when a nerve is cut the blood- 

 vessels which it supplies, after first undergoing paralytic 

 dilatation, acquire a certain degree of constriction. 



Vaso-motor Reflexes 



We now pass on to consider under what conditions these 

 efferent mechanisms are brought into play. The vaso- 

 motor centre or centres can influence, in two directions, 

 the outflow of blood from the arteries. The tonus of the 

 arterioles may be increased throughout the greater part 



