142 THE VASCULAR SYSTEM 



pressure in the arm placed at the heart's apex level. In the leg 

 at a level 380 mm. below the arm 90 mm. H.,0 sufficed, giving 

 a difference of 70 mm. H 2 against a gravity difference of 

 380 mm. H 2 0. On supplying the armlet and raising the pressure 

 in it to 910 mm. H 2 0, the capillary pressure in the forearm rose to 

 45 mm. H 2 0, at a time when the venous pressure had risen to 

 that in the armlet. On another occasion it was 57 mm. H 2 0, 

 and in the armlet and veins 910 mm. H 2 0. 



These observations only prove what any one can feel for him- 

 self by holding a limb motionless and dependent for some time 

 and then estimating with his finger the pressure in the veins and 

 the capillary areas of the skin. Every movement that occurs under 

 the varying conditions of active life squeezes on the capillary and 

 venous blood. The tissues and pressure sense organs are thus 

 protected from any great increase of vascular pressure. After an 

 area of capillaries has been squeezed empty, the blood pressure 

 for a. time must be nil within it. The blanching effect of clenching 

 the fist demonstrates this. Thus in spite of the hydrostatic pressure 

 due to gravity, equal to 140 mm. Hg in a man six feet high standing 

 vertical, in spite of this the same bristle will blanch the capillaries 

 of his feet and hand, so long as he does not keep the parts for long 

 immobile, but by contractions of his muscles every now and then 

 expresses the blood from the capillaries onward past the venous 

 valves. The vaso-motor system by constricting the arterioles 

 the arteries constrict of themselves to increased pressure prevents 

 over rapid filling of dependent parts, so that it takes some little 

 time for the dependent and warm immobile hand to become greatly 

 congested, and still longer the foot, while in a cold atmosphere, 

 owing to vaso-constriction, congestion is only very slowly pro- 

 duced by the dependent posture. 



So that while arterial and venous pressure in a relaxed limb 

 exactly follow the variations of hydrostatic pressure on change of 

 posture, the capillaries do not. Likewise on obstruction of the 

 veins, while the venous pressure rapidly rises to the pressure of 

 the obstructing armlet, the capillary pressure rises only very 

 slowly. This fact which seems at first sight contrary to physical 

 laws, in that the pressure is high in the artery which feeds and in 

 the vein which drains but not in the capillary area which joins 

 together the two, this fact is explained if we suppose that the 

 capillary bed is a sponge work emptied by every movement, and 



