^> 



BLOOD.] PHYSIOLOGY, 47 



a few hours or for a day, you would find, instead of 

 the large jelly quite filling the pail, a smaller but 

 firmer jelly covv^red by or floating in a colourless or 

 very pale yellow liquid. This smaller, firmer jelly, 

 which in the course of a day or so would get still 

 firmer and mailer, would in fact go on shrinking in 

 size, you may still call the clot ; the clear fluid in 

 which it is floating is called serum. 



What has taken place is as follows. Soon after 

 blood is shed there is formed in it a something which 

 was not present in it before. This something, which 

 we call fibrin, starts as a multitude of fine tender 

 threads which run in all directions through the mass 

 of blood, forming a close network everywhere. So 

 the blood is shut up in an immense number of little 

 chambers formed by the meshes of the fibrin ; and 

 it is this which makes it sc?m a jelly. But each th. ^d 

 of fibrin as soon as it is formed begins to shrink, and 

 the blood in each of these little chambers is squeezed 

 by the shrinking of its walls of fibrin, and tries to 

 make its way out. The corpuscles get caught in the 

 meshes, but all the rest of the blood passes between 

 the threads and comes out on the top and sides of 

 the pail. And this goes on until you have left in the 

 clot very little besides corpuscles entangled in a net- 

 work of fibrin, and all the rest of the blood has been 

 squeezed outside the clot, and is then called serum. 

 Serum, then, is blood out of which the cor- 

 puscles have been strained by the process 

 of clotting. - V ' 



Now I dare say you are ready to ask the question, 

 If blood clots so readily when it is shed, why does it 

 not clot inside the body? Why is our blood ever 



