76 SCIENCE PRIMERS, [§ v. 



far the most important are the valves in the heart : 

 they do the chief work ; those in the veins do little 

 more than help. ' 



33. Wei], then, we understand now, do we not ? why 

 I the blood, if it moves at all, moves in the one way 

 only. There still remains the question. Why does 

 the blood move at all ? 



I You know that during life it does keep moving. 



. You have seen it moving in the web of a frog's foot^ — 



land whenever any part of the body can be brought 



i under the microscope, the same rush of red corpuscles 



' through narrow channels may be seen. You know it 



! ■ moves because when you cut a blood-vessel the blood 



runs out. If you cut an artery across, the blood 



gushes out from the end which is nearest the heart ; if 



you cut a vein across, the blood comes most from the 



end nearest the capillaries. If you want to stop an 



{artery bleeding, you tie it between the cut and the 



heart ; if you want to stop a vein bleeding, you tie it 



between the cut and the capillaries. You understand 



now why there is this difference between a cut artery 



and a cut vein. And you see that this is by itbelf a 



proof that the blood moves in the arteries from the 



heart to the capillaries, and in the veins from the 



j caDillaries to the heart. ,.., ; , <, 



I The blood is not only always moving, but moves 



very fast. It flies along the great arteries at perhaps 



ten inches in a second. Through the little bit of 



capillaries along which it has to pass it creeps slowly, 



but manages sometimes to go all the way round from 



vein to vein again in about half a minute. 



It is always moving at this rapid rate, and when it 

 ceases to move, you die. ri . -. 1 -: i ; .;. 



' -., 



