138 THE CLOTTING OF BLOOD, 



fat. In addition to these nourishing substances, the 

 blood which flows away from muscle or gland or brain 

 contains some waste substances, which it is carrying off 

 to the lungs or kidneys for removal from the body. 



11. The Clotting or Coagulation of Blood. When blood 

 is first drawn from the living body it is perfectly liquid, 

 flowing in any direction as easily as water. Very soon 

 it becomes thick and sticky, like a red syrup; and at the 

 end of five or six minutes it " sets" into a stiff jelly, which 

 sticks to the cup or basin in which the blood is contained, 

 so that the vessel may be turned upside down without 

 spilling a drop. This alteration of the blood is named 

 clotting or coagulation. It is due to a change of some of 

 the dissolved albumens of the blood, into a solid sub- 

 stance named fibrin. 



If the jelly be kept for half an hour or so, it shrinks 

 and squeezes out a liquid named serum. The solid part 

 floats in the serum and is named the clot. 



12. The Use of Coagulation is to save us from the risk of 

 bleeding to death from wounds. So long as the blood is 

 flowing in healthy living blood-vessels, no fibrin forms in 

 it, and it does not clot. But as soon as blood gets out- 

 side of the vessels, or whenever their lining is injured, 

 clotting takes place. In this way, the ends of the 

 small blood-vessels in a cut finger are soon clogged up, 

 if we can only stop the flow for a little and give time 

 for a clot to form in them. 



11. What is the consistency of fresh blood ? What changes occur 

 in it during the first five or six minutes after it is drawn ? What is 

 the solidifying of the blood called ? To what is it due ? What is 

 serum ? What is the clot ? 



12. Use of coagulation ? When does it not occur ? When does it 

 take place? Why does a cut finger stop bleeding after a short time ? 



