CHAP. X.] THE VASCULAR SYSTEM. Ill 



fourteenth year to 80 ; and is reduced to the adult standard by 

 the twenty-first year. At every age, mental excitement may 

 produce a temporary acceleration, varying in degree with the 

 ^peculiarities of the individual. 



As a rule, the rapidity of the heart's action is in inverse ratio 

 to its force. A slow pulse, within physiological limits, is 

 usually a strong one, and a rapid pulse comparatively feeble. 

 The same is true in disturbance of the heart's action in disease ; 

 the pulse in fever, or other debilitating affections, becoming 

 weaker as it grows more rapid. 



Arterial tension. When an artery is severed, the flow of 

 blood from the proximal end (that on the heart side) comes in 

 jets corresponding to the heart-beats, though the flow does not 

 cease between the beats. The larger the artery, and the nearer 

 to the heart, the greater the force with which the blood issues, 

 and the more marked the intermittence of the flow. 



When a corresponding vein is severed, the flow of blood, 

 which is chiefly from the distal end (that away from the heart), 

 is not intermittent, but continuous ; the blood comes out with 

 comparatively little force, and " wells up," rather than " spurts 

 out." 



The continuous, uninterrupted flow of blood in the veins is 

 caused by the elasticity of the arterial walls. On account of the 

 small size of the capillaries and small arteries the blood meets with 

 a great deal of resistance in passing through them; and, in conse- 

 quence, the blood cannot get through the capillaries into the veins 

 so rapidly as it is thrown into the arteries by 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 following illus- 

 tration will explain how the elasticity of the arteries 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. 



