THE CIRCULATION 97 



also how the expansion and contraction of the lungs act as 

 a great pump ; and how these aids, together with changes of 

 posture, enable the blood to reach the heart again. If 

 one stands perfectly still for some time, the blood, owing 

 to its weight and the lack of pressure on the veins, slowly 

 congests in the veins in the lower part of the body, and 

 the consequences may be serious. 



159. Hence the pressure is greatest in the arteries, less 

 in the capillaries, and least of all in the veins. 



160. How the Amount of Blood flowing through any 

 Organ may be modified. We learned that the ability to 

 do this lies in the muscular or middle coat of the vessels, 

 and that the muscular coat in turn is controlled by the 

 nerves. This very important power, therefore, is intrusted 

 to the two master tissues. The nerves that control the 

 sizes of the blood vessels are called vasomotor nerves and 

 are of two kinds. One kind, the constrictor nerves, stimu- 

 lates the walls of the blood vessels to contract, while the 

 other kind of vasomotor nerves, called the dilator nerves, 

 neutralizes or inhibits the effect of the constrictors, and 

 thus allows the blood tubes to enlarge. The regulation 

 is involuntary, or beyond the control of the will; for in- 

 stance, the blood vessels of the brain may become enlarged 

 and the great pressure there cause a headache, but the 

 will cannot drive it away. We sit before a fire, and the 

 face becomes red as the warmth soothes the constrictor 

 nerves into inactivity ; or the constrictors leading to the 

 face may become paralyzed by mental confusion and we 

 blush. 



161. Why must the Relative Amount of Blood in the 

 Different Organs continually change ? The necessity for 

 this comes from the fact that if the one and one fourth 

 gallons of blood were evenly distributed, none of the 

 organs would be capable of any powerful and effective 

 action. A person weighing 157 pounds has only 12 



