112 ANATOMY FOR NURSES. [CHAP. X. 



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 quantity of blood out of the capillaries ; 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 elasticity, and so are pressing the blood on in a steady 

 stream into the capillaries with as much force as they were 

 themselves distended 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- 

 ing application, the blushing or paling of the face from mental 

 emotion, the increased flow of blood to the mucous membranes 

 during digestion, being all instances of this kind. 



But the condition of the capillary walls themselves also 

 exerts an influence upon the capillary circulation. If some 

 transparent tissue, preferably the web of a frog's foot, be watched 

 under the microscope, it will be observed that in the small 

 capillaries the corpuscles are pressed through the channel in 

 single file, each corpuscle as it passes occupying the whole bore 

 of the capillary. In the larger capillaries and smaller arteries 

 and veins the red corpuscles run in the middle of the channel, 

 forming a coloured core, between which and the sides of the 

 vessels is a colourless layer containing no red corpuscles, and 

 called the "peripheral zone." In the peripheral zone are fre- 



