EXERCISE 76 



Problem. What takes place when the blood clots ? 



What to use. Fresh blood from the butcher or the slaughterhouse, to which a few drops of for- 

 malin have been added as a preservative ; two glass beakers or battery jars with covers ; an egg-beater 

 or a bundle of thin twigs ; water ; covers for jars. 



What to do. Place equal quantities of blood in the two vessels. Set one (A) aside, covered, to 

 remain undisturbed. In the other (B) beat up the blood with the egg-beater or the twigs. Draw out 

 what clings to the beater after a few minutes. Cover and set vessel B aside to remain undisturbed. 



Wash the material that clings to the beater in fresh water. On the following day examine the con- 

 tents of A and B. 



Record. Describe the stuff that has been removed from the blood by the beating. What changes 

 have taken place in A ? Make a diagram to show results. What has happened in B ? 



Questions. 1. What is there in A that was not there the day before? 



2. Where did it come from ? 



3. Remove the clot from A and wash in fresh water ; what change does the washing bring about ? 



4. What is now absent from A that was there on the previous day ? What has become of it ? 



NOTE. The shiny stuff removed from the blood by beating is fibrin. This forms the clot. Plasma contains a substance 

 called fibrinogen ; when this is brought in contact with the air, it becomes fibrin. Beating brings air in contact with all parts 

 of the plasma and removes the fibrin as fast as it is formed. The clear fluid in the jar after the clot is formed is called serum. 



5. What has become of the corpuscles in A ? 



Blood = plasma + corpuscles 

 Plasma = serum + fibrinogen 



6. Why do small cuts usually stop bleeding in a short while ? Why do they sometimes continue 

 to bleed for a long while ? 



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