240 THE HUNAN BODY. 



.jejrial pressure in all parts nearer the heart, while dilatation 

 of the small arteries will have the contrary effect. 



Summary. "Wo find then that arterial pressure at any 

 moment is dependent upon (1) the rate of the heart's beat;, 

 (2) the quantity of blood forced into the arteries at each 

 beat; (3) the calibre of the smaller vessels. All of these, 

 and consequently the capillary circulation which depends; 

 upon arterial pressure, are under the control of the nervous 

 system (see Chap. XVII.). 



The Pulse. When the left ventricle contracts it forcea 

 a certain amount of blood into the aorta, which is already 

 distended and on account of the resistance in front cannot 

 empty itself as fast as the contracting ventricle fills it. As 

 a consequence its elastic walls yield still more it enlarges 

 both transversely and longitudinally and if exposed in a 

 living animal can be seen and felt to pulsate, swelling out 

 at each systole of the heart, and shrinking and getting rid 

 of the excess during the pause. A similar phenomenon, 

 can be observed in all the other large arteries, for just as 

 the contracting ventricle fills the aorta faster than the 

 latter empties (the whole period of diastole and systole 

 being required by the aorta to pass on the blood sent in 

 during systole) so the increased tension in the aorta im- 

 mediately after the cardiac contraction, drives on some 

 of its contents into its branches, and fills these faster than. 

 they are emptying and so causes a dilatation of them also, 

 which only gradually disappears as the aortic tension falls 

 before the next systole. Hence after each beat of the heart 

 there is a sensible dilatation of all the larger arteries, known 

 as the pulse, which becomes less and less marked at points 

 on the smaller branches farther from the heart, but which in 

 health can readily be recognized on any artery large enough 

 to be felt by the finger through the skin, etc. The radial 

 artery near the wrist, for example, will always be felt tense 

 by the finger, since it is kept overfilled by the heart in the 

 way already described. But after each heart-beat it be- 

 comes more rigid and dilates a little, the increased disten- 

 sion and rigidity gradually disappearing as the artery 

 passes on the excess of blood before the next heart-beat. 



