THE CIRCULATION OF THE BLOOD 187 



This elasticity of the arterial wall, in response to 

 the pressure and reaction of the column of blood 

 imparted to it by the heart muscle, develops into a 

 remittent expansion and recoil of the arterial wall, 

 which becomes fainter the more distant the vessels 

 are from the heart. When the capillaries are reached 

 it is a continuous or a steady flow of blood, without 

 any recoil of the arterial wall, which passes into the 

 veins. Thus the elasticity of the arteries is for the 

 purpose of equalizing the movement of the blood 

 throughout the arterial system. 



Contractility. Contractility of the arteries is depen- 

 dent upon the muscular tissue in their walls. They 

 are supplied by nerve filaments which receive impulses 

 from the controlling centres in the spinal cord, which 

 communicate by means of ganglia with the sympathetic 

 system, and the latter distributes filaments to the 

 middle or muscular coat of the arteries. The centres 

 in the spinal cord are called the vasomotor centres, and 

 the nerve filaments the vasomotor nerves. The centres 

 in the spinal cord are influenced chiefly by a main 

 centre situated in the medulla; in other words, the 

 spinal centres are underlying centres of this system. 



The vasomotor nerves possess two sets of fibers: 

 those which when stimulated raise the blood pressure 

 or contract the arterial wall called vasoconstrictor 

 nerves; and those which lower arterial pressure or 

 dilate the arterial wall called vasodilator nerves. 

 These two sets of nerves, both when active although 

 antagonistic to each other in function, tend to keep 

 the arteries in a normal state of contraction, thus 

 regulating the blood pressure and caliber of the 

 arteries. 



The vasa vasorum is the term defining the blood- 

 vessels which supply the walls of the arteries. They are, 

 of course, very minute bloodvessels which nourish 

 them, and are derived from adjacent branches and have 

 no direct opening into the channel of the artery they 



