THE BLOOD 121 



at a very low pressure. This is exactly the 

 condition most favourable for the transuda- 

 tion of fluid through the thin walls of the 

 capillaries for the nourishment of the living 

 tissues. But there is another remarkable 

 arrangement that suits two purposes : the 

 muscular w r alls of the arterioles, by contracting, 

 can vary the diameter of these small vessels. 

 When the calibre is diminished, it will be 

 evident that the blood will not pass through 

 the small vessels so easily as it will do when 

 the calibre is increased. The contractile 

 arterioles act like a kind of stop-cock at one 

 part of the system. When the stop-cock is 

 open, as when the arterioles are dilated, 

 the blood flows through easily, the arterial 

 system empties quickly through the capillaries 

 into the veins, and the pressure in the greater 

 arteries falls. When the stop-cock is partly 

 closed, the blood will meet with resistance, 

 and the pressure in the larger arteries rises. 

 Thus, as the arterioles are always partially 

 contracted under the influence of special 

 nerves, there is always a sufficiently high 

 pressure in the arterial system to keep up the 



