46 



SCIENCE PRIMERS. 



[8 IV. 





it does not ; and if you keep on stirring long enough 

 you will find that it never clots at all. By conti- 

 nually stirring it you will prevent its clot- 

 ting. Now take out your bundle of twigs : you will 

 find it covered all over with a thick reddish mass of 

 some soft sticky substance ; and if you pump on the 

 red mass you will be able to wash away all its red 

 colour, and will have nothing left but a quantity of 

 white, soft, sticky, stringy material, all entangled and 

 matted together among the twigs of your bundle. 

 This stringy material is in reality made up of a number 

 of fine, delicate, soft, elastic threads or fibres, and is 

 called fibrin. 



You see, by stirring, or, as it is frequently 

 called, whipping the blood with the bundle 

 of twigs, you have taken the fibrin out of 

 the blood, and so prevented its clotting. 



If you were to take one of the clotted lumps of 

 blood that were spilt on the ground or a bit of the clot 

 from a pail in which the blood had not been whipped, 

 and wash it long enough, you would find at last that 

 all the colour went away from the lump, and you had 

 nothing left but a small quantity of white stringy sub- 

 stance. This white stringy substance is fibrin — exactly 

 the same thing you got on your bundle of twigs. 



If the blood is carefully caught in a pail, and after 

 wards not disturbed at all, it clots into a solid mass. 

 The whole of the blood seems to have changed into 

 a complete jelly ; and if you turn it out of the pail, as 

 you may do, it keeps its shape, and gives you quite a 

 mould of the pail, a great trembling red jelly just the 

 shape of the inside of the pail. 



But if you were to leave the blood in the pail for 



