x STRUCTURE OF BLOOD-VESSELS 99 



inli-i mi and superior vena <a\a taken together in a second. 

 All the capillaries put together hold a great deal more blood 

 than the aorta, because the bore of all the capillaries put to- 

 gfthcr forms an area very much greater than that of the bore of 

 the aorta. Hut the same quantity of blood passes along the bore 

 of the aorta in a second as passes through the very much 

 wider bed formed by all the capillaries in a second, and in 

 order to do this the blood rushes along the aorta much faster 

 than it docs along the capillaries. For a like reason when a 

 river opens into a lake the velocity of the stream greatly falls 

 in the lake, but increases again in the river leading out of the 

 hike, the same quantity of water passing through the lake < 

 as rushes along the river in a given time. The velocity of the 

 blood, then, is greatest in the aorta, less in the smaller arteries, 

 least in the capillaries, and increases again in the veins. 

 In the vena.' cava? close to the heart it is less than in the aorta, 

 because the bore of these veins taken together is greater than 

 that of the aorta. The velocity of blood in the aorta is 

 ' about i 5 inches per second, in the capillaries probably less 

 than Jjfth of an inch per second, and in the inferior vena 

 cava about half what it is in the aorta. 



The Valves of Veins 



Exercise the arm for a few minutes to increase the circula- 

 tion in it ; the veins on the front of the forearm will then be 

 prominent. The veins, looked at through the skin, appear 

 bluish in colour, but the blood in them is dark reddish-purple. 

 With the finger stroke one or other of the veins upwards towards 

 the shoulder. You will find that you can press much of the 

 blood out of it. Now stroke the vein downwards towards the 

 hand ; you cannot press the blood out of it, and the vein 

 swells up and distends, the swelling being greatest at one or 

 two places along the vein, thus forming "knots." These 

 knots mark the position of valves which prevent the return 

 of the blood towards the hand. The tfeins are freely con- 

 nected with one another, so that when two or three veins 

 are compressed, the blood passes up from the limb by other 

 vessels, especially by those deeper in the arm. 



