1052 PHYSIOLOGY 



so that the output of this organ is increased and the arterial blood 

 pressure is raised. 



Since the blood in the vessels is subject to the influence of gravity, 

 we should expect to find that the pressure in the veins of the foot was 

 equal to the pressure in the veins, say, of the hand at the level of the 

 heart plus the pressure equivalent to the column of blood between 

 these veins and the heart, i.e. about a metre of blood. On measuring 

 the pressure by von Recklinghausen's or by Hill's method in these 

 veins, this is not found to be the case. The pressure, indeed, in the 

 veins of the foot is but little higher than that in the veins of the hand. 

 Von Recklinghausen found that after subtracting the distance between 

 the foot and the heart the pressure in the veins was negative by as 

 much as 40 cm. In the same way, as Hill has shown, the pressure in 

 the capillaries of the foot is about the same as in the capillaries of the 

 hand. When a man assumes the upright position the arteries of the leg 

 and foot contract until, under the combined influence of the heart's 

 contraction and gravity, the blood-supply to the capillaries is only 

 sufficient to keep the pressure in these vessels at a certain moderate 

 height. The return of the blood from the dependent parts cannot be 

 ascribed to the heart-beat at all, but is due to the extrinsic mechanism 

 of circulation through the veins, i.e. the contractions of the muscles of 

 the limb which press all the deep and superficial veins, and in virtue of 

 the valves force the blood contained therein by Poupart's ligament 

 into the abdomen. The fact that circulation through the legs is de- 

 pendent on the contractions of their muscles explains why it is so 

 difficult to stand still for any length of time without moving, and 

 emphasises the need of moderate exercise for the maintenance of a 

 normal circulation. 



