216 VASCULAR CONSTRICTION AND DILATION. [BOOK I. 



the bat's wing, and elsewhere, and similar rhythmic variations, also 

 possibly due to active rhythmic contractions, but possibly also of 

 an entirely passive nature, have been observed in the portal veins, 

 very little is known of any nervous arrangements governing the 

 veins. When in the frog the brain and spinal cord are destroyed, 

 very little blood comes back to the heart as compared with the 

 normal supply, and the heart in consequence appears almost blood- 

 less and beats feebly. This has been interpreted as indicating the 

 existence of a normal tone in the veins dependent on the central 

 nervous system. "When the latter is destroyed, the veins become 

 abnormally distended and a large quantity of blood becomes lodged 

 and hidden as it were in them. 



The Effects of Local Vascular Constriction or Dilation. 



Whatever be determined ultimately to be the modus operandi 

 of vaso-motor mechanisms, the following fundamental facts remain 

 of prime importance. 



The tone of any given vascular area may be altered, positively 

 in the direction of augmentation (constriction), or negatively in 

 the way of inhibition (dilation), quite independently of what is 

 going on in other areas. The change may be brought about by (1) 

 a stimulus applied to the spot itself, and acting either directly on 

 some local mechanism, or indirectly by reflex action through the 

 general central nervous system ; (2) by a stimulus applied to some 

 other sentient surface, and acting by reflex action through the 

 central nervous system ; (3) by a stimulus (chemical, arising in or 

 carried by the blood) acting directly on the central nervous system ; 

 (4) by some part of the central nervous system acting on the vaso- 

 motor centre, as in emotions. 



The effects of local dilation are local and general. 



The local effects are as follows. The arteries in the area being 

 dilated, offer less resistance than before to the passage of blood. 

 Consequently, more blood than usual passes through them, filling 

 up the capillaries and distending the veins. Owing to the diminu- 

 tion of the resistance, the fall of pressure in passing from the 

 arteries to the veins will be less marked than usual ; that in the 

 small arteries themselves will be lowered ; that in the corresponding 

 veins heightened. The lowering of the pressure in the arteries 

 means that their elastic coats are not put to the stretch as much 

 as usual ; i.e. their elasticity is not called into play to the same 

 extent as before. Now, as has been seen, every portion of the 

 arterial wall has its share in destroying the pulse by converting the 



