130 THE VASCULAR SYSTEM 



changes from the flushed to the blanched state. The vaso-con- 

 strictor control of the hand (for changes of posture) seems to be 

 much less than that of the foot. If in a warm atmosphere foot and 

 hand be lowered together from the heart's apex level and be kept 

 still in the dependent posture, it will be seen that congestion arises 

 much less quickly in foot than in hand, and the pressure in the 

 veins on the back of the hand reaches the full gravity effect much 

 sooner than in the veins on the dorsum of the foot. It is the same 

 with the face. The parts always exposed to the atmosphere 

 congest quickest. The control of course varies with the effect of 

 external temperature on vaso-dilatation, as may be seen by plung- 

 ing the hand and foot in iced or in hot water and then repeating 

 the observation. The pumping action of the movements of walk- 

 ing may be observed very well on the veins of the back of the 

 foot. Let the reader stand still with the feet bare, and observe 

 the veins of his feet becoming prominent. If he bend to feel them 

 he can estimate the high pressure within. Now let him take a 

 few steps and observe the veins again. They are emptied by the 

 movements, squeezed between skin and muscle, and feel soft for 

 some little time until they fill again on standing still. Similarly 

 the contractions of the muscles which occur with slight changes 

 of posture empty the veins and prevent congestion when we sit 

 working at a desk. It is the deficient flow or quality, not the 

 pressure, of blood which leads to oedema, degeneration, distension 

 of the venous wall, and varicose veins in those whose occupation 

 requires them to stand for long periods of time. Some attention 

 has been drawn to a curious hereditary cedema of the legs which 

 commences about the seventeenth year. It occurs in members 

 of certain families on standing, and is prevented by bandaging. 

 The venous pressure is raised, owing to vaso-dilatation, by warmth, 

 rest, sleep or food, together with the pressure in the arterioles as 

 measured in the distal phalanx, while the pressure in the large 

 arteries falls. The arterial and venous pressures in the arm are 

 both raised during muscular effort, owing to increased ventricular 

 output, splanchnic constriction, and local dilatation. 



THE CAPILLARY PRESSURE 



As much light can be thrown upon the principles which govern 

 the circulation of the blood, from the comparative structure of 



