THE COURSE AXD PHYSICS OF THE CIECCLATIOX 239 



above the heart has a reduced pressure because of gravity. 

 Suppose that a man has been lying down and that the 

 average pressure in the root of his aorta is 125 mm. of 

 mercury. He rises to his feet. The aortic pressure may 

 change somewhat but we will assume that it does not. 

 A moment before the arterial pressure in his head and his 

 feet was nearly equal; now the pressure in the arteries of 

 his scalp is reduced by about 50 mm. and that in his 

 feet is increased by some 90 mm. Still the heart pressure 

 is ample to maintain circulation through the brain and 

 the arteries of the legs are not capacious enough to 

 accumulate much extra blood. 



In a three-story house we want to have water pressure 

 enough to provide for a flow on all the floors at once but 

 we cannot avoid having a greater pressure in the base- 

 ment than in the attic. On the whole it is remarkable 

 that the body can make such changes of position and 

 still be well served by the circulation. When there is 

 faintness, the influence of gravity is distressingly ap- 

 parent. The usual cause of faintness is an insufficient 

 blood-supply to the brain associated with a failure of the 

 normal surplus pressure in the arteries of the head. It is 

 then of great advantage to lie down and bring these ves- 

 sels into a position where they are no longer subjected 

 to a handicap. 



A most striking effect of gravity is to create an ex- 

 ception to the rule that venous pressure is always low. 

 The blood in the veins of the legs and feet, when the 

 subject is standing, exerts a very considerable pressure 

 upon the confining vessels. In the feet this may ap- 

 proach 100 mm. of mercury, an amount which is almost 

 of the general arterial order. It is not surprising that 

 x>se veins develop in the lower extremities. 



The Portal System. According to our description, as 



far as we have gone, the blood which is sent out from the 



left ventricle is forced through one set of capillaries and 



then returns to the right side of the heart by way of the 



ernic veins. We must now indicate an important 



