298 CHANGES IN QUANTITY OF BLOOD. [BOOK i. 



of the internal organs), which are abnormally distended to contain 

 the surplus. 



We learn, also, from these facts the two practical lessons : first, 

 that blood pressure cannot be lowered directly in a mechanical 

 manner by bleeding, unless the quantity removed be dangerously 

 large ; and secondly, that there is no necessary connection between 

 a high blood pressure and fulness of blood or plethora, since an 

 enormous quantity of blood may be driven into the vessels without 

 any marked rise of pressure. 



When a quantity of blood or, indeed, of fluid is injected into 

 the veins, the output from the heart is increased and observations 

 seem to shew that the increase in the output is out of proportion 

 to the quantity of fluid injected, indicating that the result is of 

 complex origin. In spite of this increased output, the blood 

 pressure is not raised ; the effect is compensated by vascular 

 dilation somewhere. Conversely when blood is withdrawn, the 

 output is diminished, but here again the effect on the blood 

 pressure is soon compensated, this time by vascular constriction. 



