THE BLOOD 39 



minutes, tin two lots of blood may be taken away for subse- 

 quent observation. 



I'.lood freshly drawn is jwrfectly fluid. After it has been 

 standing f>r two or three minutes it becomes " thick " or viscid ; 

 this will be readily noticed if the basin containing it is tilted. 

 Three or four minutes later still the whole sets to a jelly, and 

 this jelly becomes subsequently so firm that the vessel if small 

 can be turned upside down without the blood falling out. After 

 a time, it may be an hour or more, a few drops of a yellow fluid 

 seem to have oozed out from the jelly-like mass, or clot The 

 surface of the clot will be concave, because while it is shrinking, 

 its edges adhere at first to the sides of the basin. Gradually 

 more and more of the yellow fluid, serum, collects at the 

 top and sides. The serum is squeezed out of the clot as the 

 clot goes on shrinking, until at length there is a considerable 

 amount of serum with a red clot lying in it. 



If a drop of the serum be carefully removed and examined 

 with the microscope, no red or even colourless corpuscles will be 

 found in it. If a particle of the clot be flattened out and 

 examined in the same way, crowds of red corpuscles and some 

 colourless ones, with a close meshwork of fine threads binding 

 them together, will be seen. The close meshwork consists of 

 fine fibrils of a substance called fibrin. The clotting of the 

 blood, as this process is called, leads then to the formation of 

 a fluid, serum, and of a clot consisting of fibrin and the red 

 and colourless corpuscles. 



\Vln-n blood, as sometimes happens, is a long time in clotting, 

 some of the corpuscles have time to sink, and since the red ones 

 sink more easily than the colourless ones, the uppermost layer of 

 the clot is formed chiefly of colourless corpuscles, and hence is 

 lighter in colour than the rest of the clot. This layer is called 

 the buffy coat. 



If fresli blood is rapidly stirred with a bundle of twigs, as 

 directed above, the threads of fibrin as they form are caught 

 up and cling to the twigs ; and as the formation of the fibrin 

 goes on the threads become matted up on the twigs, and so, 

 after the stirring has been continued for three or four minutes, 

 all the fibrin which can be formed will be formed, and will 

 have been caught up and collected on the twigs. If the twigs 

 are then washed in a stream of water, nearly pure fibrin will 



