134 ANATOMY FOR NURSES. [Chap. XL 



the heart. The whole arterial system, therefore, becomes over- 

 distended with blood, and the greater the resistance, the greater 

 the pressure on, and distension of, the arterial walls. The fol- 

 lowing illustration will explain how the elasticity of the arter- 

 ies enables them to deliver the blood in a steady flow to the 

 veins through the capillaries. 



If a syringe be fastened to one end of a long piece of elastic 

 tubing, and water be pumped through the tubing, it will flow 

 from the far end in jerks. But if we stuff a piece of sponge 

 into this end of the tubing, or offer in any way resistance to 

 the outflow of the water, the tubing will distend, its elasticity 

 be brought into play, and the water flow from the end not in 

 jerks, but in a stream, which is more and more completely con- 

 tinuous the longer and more elastic the tubing. 



Substitute for the syringe the heart, for the sponge the cap- 

 illaries and small arteries, for the tubing the whole arterial sys- 

 tem, and we have exactly the same result in the living body. 

 Through the action of the elastic arterial walls the separate jets 

 from the heart are blended into one continuous stream. The 

 whole force of each contraction of the heart is not at once 

 spent in driving a certain quantity of blood onwards ; a part 

 only is thus spent, the rest goes to distend the elastic arteries. 

 But during the interval between that beat and the next, the 

 distended arteries are narrowing again, by virtue of their elas- 

 ticity, and so are pressing the blood on in a steady stream into 

 the capillaries with as much force as they were themselves dis- 

 tended by the contraction of the heart. 



The degree of tension to which the arterial walls are sub- 

 jected depends upon the force of the heart-beat, and upon the 

 resistance offered by the smaller arteries, the normal general 

 blood pressure being mainly regulated by the " tone " of the 

 minute arteries. 



Variations in the capillary circulation. — Most of the changes 

 in the capillary circulation are likewise dependent upon the 

 condition of the smaller arteries. When under certain nervous 

 influences they contract, the blood supply to the capillaries is 

 greatly lessened ; when, on the other hand, they dilate, the 

 blood supply is greatly increased. The phenomena produced 

 by these local variations in the blood supply of certain parts are 

 very familiar to us ; the redness of skin produced by an irritat- 



