THE CIRCULATION 159 



we come to consider the relation of respiration to the 

 circulation. 



The squeezing 1 of the veins by contraction of the 

 skeletal muscles is also a factor of great importance 

 in promoting the venous flow, and is one of the chief 

 reasons why muscular exercise so powerfully promotes 

 the circulation. Whilst one is standing this factor is 

 in abeyance, and the venous flow in the legs is sluggish ; 

 whereas during walking the veins are constantly being 

 massaged by the contractions of the muscles of the legs, 

 and the return of blood from them is brisk. It is for 

 this reason that much standing tends to produce varicose 

 veins, whilst walking does not. The military plan of 

 ' marking time ' when standing tends to counteract the 

 tendency to stagnation of blood in the veins of the legs. 



The pressure exerted by the ' tone ' of the skin and 

 tissues is only second in importance to the more active 

 squeeze effected by the muscles. Where this tone is 

 low, as in the scrotum, overdistension and varicosity of 

 the veins easily occurs, and it used to be a recognized 

 method of treating varicose veins in the leg to excise 

 a large area of skin on the back of the calf, in order 

 to increase the pressure on the veins by forming a 

 natural ' elastic stocking.' 



Blood-pressure. 



Blood-pressure signifies the degree of fluid-pressure 

 present within the vessels, putting their walls on the 

 stretch. As commonly used, the expression refers only 

 to the arterial blood-pressure, and as this is the sense in 



