BLOO^.J PHYSIOLOGY. 45 



a rounded square, and then into the shape of a pear, 

 and then into a thing that had no shape at all, and 

 then back again into a round ball, and kept doing 

 this apparently all of its own accord while you were 

 looking at it — wouldn't you think it very curious? 

 Well, one of these little bodies in the blood of which 

 we are speaking, and which are called white corpuscles, 

 may be seen, when a drop of blood is watched under 

 the microscope, to go on in this way, continually 

 changing its shape. But of these white corpuscles 

 of the blood, and of their wonderful movements, you 

 will learn more as you go on in your physiologica! 

 studies. 



23. Besides these red and white corpuscles there is 

 nothing else very important in the blood that you can 

 see with the microscope ; but their being in the 

 blood is one reason why blood is thicker than water. 



Did you ever see a pig or sheep killed ? If so, you 

 would be sure to notice that the blood ran quite fluid 

 from the blood-vessels in the neck, ran and was spilt 

 like so much water — ^but that very soon the blood 

 caught in |the pail or spilt on the stones became quite 

 solid, so that you could pick it up in lumps. When- 

 ever blood is shed from the living body, within a short 

 time it becomes solid. This becoming solid is called 

 the clotting or coagulation of blood. 



What makes it clot ? Suppose while the blood was 

 running from the pig's neck into the butcher's pail, and 

 while it was still quite fluid, you were to take a bunch 

 of twigs and keep slowly stirring the blood round and 

 round in the pail. You would naturally expect that 

 the blood would soon begin to clot, would get thicker 

 and thicker and more and more difficult to stir. But 



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