934 PHYSIOLOGY 



the heart, i.e. about a metre of blood. On measuring the pressure by 

 von Kecklinghausen'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 Kecklinghausen 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 contrac- 

 tions 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 dependent 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. 



