168 UNCONSCIOUS NERVOUS OPERATIONS 



Action of the Arteries. All the blood vessels are 

 always full of blood, at the moment of the heart's pause, 

 as at every other. When, therefore, the contraction of 

 the left ventricle forces from four to six additional ounces 

 of blood into the aorta, that which already fills the vessels 

 must be crowded on. In the minute tubes of the capil- 

 laries there is, however, a very considerable amount of 

 friction to be overcome, and as beat follows beat, the 

 elastic walls of the arteries must stretch to receive the 

 flow. The elastic arteries also react upon the blood 

 between the beats to force it through the capillaries. 

 Thus the flow, which is intermittent in the arteries, 

 becomes continuous in the capillaries. 



The muscular contraction in the arteries helps to regu- 

 late the amount of blood sent to different parts at different 

 times. Many familiar facts illustrate this. Blushing is 

 due to an increased flow of blood to the face ; a mustard 

 plaster draws more blood to the area which it covers ; 

 friction of the surface has a like effect. After death the 

 arteries are always found empty, their last contraction 

 having forced the blood into the veins. 



233. Blood Pressure. During life the elastic walls of 

 the arteries are always distended, and the pressure upon 

 their walls of the extra quantity of blood forced into 

 them by the beating of the heart is called blood pres- 

 sure. When an artery is cut, it is noticed that the blood 

 issues from it in jets corresponding to the beats of the 

 heart, and the nearer the cut is -to the heart, the stronger 

 is the spurt. When a vein is severed, on the other hand, 

 the flow is steady and with less force. 



The difference in the amount of blood pressure in veins 

 and arteries is shown by experiments upon animals. If a 

 long glass tube be introduced into the carotid artery of 



