138 OUR BODIES AND HOW WE LIVE 



called the serum, remains at the top. This change of 

 the blood, after it is drawn from the body, into a jelly- 

 like, semisolid mass, is called the "clotting, or the coagulation, 

 of the blood. 



The watery part, or serum as it is called, is blood from 

 which the corpuscles have been removed by the process of 

 clotting. It is largely made up of water in which albumin 

 is dissolved. 



The clot consists mainly of two substances, the cor- 

 puscles and a network of white, tough, fibrous threads, 

 called fibrin. The clotting of blood is not yet fully under- 

 stood, but it is thought to be due in part to the formation 

 of this fibrin. The corpuscles get snarled in the meshes of 

 the fibrin, and thus the clot is formed. 



194. Why the Clotting of Blood is Important. The fact 

 that blood clots when shed is one of its most striking prop- 

 erties. It is of the most vital importance. When a person 

 receives a severe wound he would bleed to death unless 

 clotting set in. Nature in this way plugs up the wound 

 with clots of blood and prevents ' too much bleeding. 

 Blood does not clot within the healthy vessels. It may do 

 so in certain diseased conditions. 1 



Experiment 40. Put two or three drops of fresh blood on the 

 bottom of a little white butter plate. Place the plate in a saucer 

 which has a little water in it and cover it with an inverted goblet. 

 Take off the cover in five minutes, and the drop has set into a jellylike 

 mass. In half an hour a little clot will be seen in the watery serum. 



1 Thus, in those who suffer from enlarged or varicose veins, a clot may 

 sometimes form in the veins of the leg. Snake poison may act to cause 

 the blood to clot within the vessels. In some very rare cases there are 

 people born in whom the blood has little or no power of clotting. These 

 are known as " bleeders," and with them a slight cut on the finger or even 

 a pin scratch may prove a serious matter, owing to the loss of blood. 



