218 VASCULAR CONSTRICTION AND DILATION. [BOOK i. 



cerebro-spinal centres, and called forth by stimuli either intrinsic 

 and acting through the blood, or extrinsic and acting through 

 nervous tracts, the supply of blood to this or that organ or tissue 

 may be increased or reduced : the surplus or deficit being carried 

 away to, or brought up from, either the rest of the body generally 

 or some other special organ or tissue. 



