THE COURSE AND PHYSICS OF THE CIRCULATION 233 



We are in possession of the following facts : pressure in 

 the arteries is high and fluctuating, slightly higher in 

 the main trunks than in their branches; pressure in the 

 veins is low and relatively constant. It must be rather 

 higher in the small veins than in the large ones which 

 they unite to form; the fact that the blood flows from the 

 smaller to the larger ones makes this certain. An im- 



FIG. 48. The modern method of measuring blood-pressure in ani- 

 mals.' A glass nozzle or cannula like (c) is tied into an artery as at (a). 

 The pressure elevates the more distant limb of a mercury column in a 

 U-tube and a float on the mercury causes a writing-point to trace upon 

 a revolving drum. Transmission from the blood to the mercury is 

 by tubing filled with a solution restraining coagulation. 



portant inference from all this is that a great fall of 

 pressure occurs in the region of the small vessels the 

 capillaries and the minute arteries and veins immediately 

 adjoining them. This we must presently account for. 



We may think of the pressure of the blood as a measure 

 of the energy applied to the blood by the ventricle. What 

 becomes of this energy? In any such system, whether 



