COAGULATION OF THE BLOOD 1 19 



and, as it becomes greater and greater in amount, to form a complete super- 

 ficial stratum above the solid clot. At the same time the serum begins to 

 transude at the sides and at the under surface of the clot, which in the course 

 of an hour or two floats in the liquid. The appearance of the serum is due 

 to the fact that the clot contracts, thus squeezing the fluid out of its mass. 

 The first drops of serum appear on the surface about eleven or twelve min- 

 utes after the blood has been drawn; and the fluid continues to transude for 

 from thirty-six to forty-eight hours. 



The clotting of blood is due to the development in the plasma of an in- 

 soluble substance called fibrin. This fibrin forms threads or strands through 

 the mass in every direction. The strands adhere to each other wherever 

 they come in contact, thus forming a very dense tangle and meshwork which 

 incloses within itself the blood-corpuscles. The clot when first formed, 

 therefore, includes the whole of the blood in an apparently solid mass, but 

 soon the fibrinous meshwork begins to contract and the serum is squeezed 

 out. When a large part of the serum has been squeezed out the clot is found 

 to be smaller, but firmer and harder, as it is now made up more largely of 

 fibrin and blood corpuscles. Thus in coagulation there is a rearrangement 

 of the constituents of the blood; liquid blood consisting of plasma and 

 blood corpuscles, and clotted blood of serum and clot. These relations are 

 roughly shown in the following diagram: 



Liquid blood. 



Plasma. Corpuscles. 



Serum. Fibrin. 



Clot. 



Clotted blood. 



The rapidity with which coagulation takes place varies greatly in different 

 animals and at different times in the same animal. Where coagulation is 

 very slow the red corpuscles, which are somewhat heavier than plasma, 

 often have time to settle considerably before the fibrin is formed. If the 

 blood is rapidly cooled to a temperature approaching o C. then the clot is 

 very greatly delayed. Horse's blood is particularly favorable for demon- 

 strating this point. In it clotting occurs so slowly that very often the red 

 corpuscles will completely settle out, and when the blood is again warmed 

 and the clotting takes place there is a superficial stratum differing in appear- 

 ance from the rest of the clot, having a grayish-yellow color. This is known 



