THE CIRCULATION 49 



flowing into the capillaries. In this way they cause the 

 intermittent flow from the heart to become a steady stream 

 in the capillaries. 



The swelling of the arteries at each contraction of the 

 ventricle is easily felt at certain places in the body, such 

 as the wrist. This expansion, known as the "pulse," is 

 the chief means employed by the physician in determining 

 the force and rapidity of the heart's action. 



Purpose of the Valves in the Veins. The valves in the 

 veins are not used for directing the general flow of the 

 blood, the valves of the heart being sufficient for this pur- 

 pose. Their presence is necessary because of the pressure 

 to which the veins are subjected in different parts of the 

 body. The contraction of a muscle will, for example, close 

 the small veins in its vicinity and diminish the capacity 

 of the larger ones. The natural tendency of such pressure 

 is to empty the veins in two directions one in the same 

 direction as the regular movement of the blood, but the 

 other in the opposite direction. The valves by closing 

 cause the contracting muscle to push the blood in one 

 direction only toward the heart. The valves in the veins 

 are, therefore, an economical device for enabling variable 

 pressure in different parts of the body to assist in the cir- 

 culation. Veins like the inferior vena cava and the veins of 

 the brain, which are not compressed by movements of the 

 body, do not have valves. 



Purposes of the Muscular Coat. The muscular coat, 

 which is thicker in the arteries than in the veins and is 

 more marked in small arteries than in large ones, serves 

 two important purposes. In the first place it, together 

 with the elastic tissue, keeps the capacity of the blood 

 vessels equal to the volume of the blood. Since the blood 

 vessels are capable of holding more blood than may be 



