180 THE HUMAN BODY. 



manent. In a few minutes the top of the jelly-like mass 

 will be seen to be hollowed or " cupped," and in the con- 

 cavity will be found a small quantity of nearly colorless 

 liquid, the Hood-serum. The jelly next shrinks so as to 

 pull itself loose from the sides and bottom of the vessel con- 

 taining it, and as it shrinks it squeezes out more and more 

 serum. Ultimately we get a solid clot, colored red and 

 smaller in size than the vessel in which the blood coagu- 

 lated, but retaining its form, and floating in a quantity of 

 pale yellow serum. The whole series of changes leading to 

 this result is known as the coagulation or clotting of the 

 blood. 



Cause of Coagulation, If a drop of fresh drawn blood 

 be spread out and watched with a powerful microscope, it 

 will be seen that its coagulation is due to the separation of 

 very fine solid threads which run in every direction through 

 the plasma and form a close network entangling all the 

 corpuscles. These threads are composed of an albuminous 

 substance known as fibrin. When they first form, the 

 whole drop is much like a sponge soaked full of water 

 (represented by the serum) and having solid bodies (the 

 corpuscles) in its cavities. After the fibrin threads have 

 been formed they begin to shorten; hence the fibrinous 

 network tends to shrink in every direction, and this shrink- 

 age is greater the longer the clotted blood is kept. At first 

 the threads stick too firmly to the bottom and sides of the 



What is meant by the stage of gelatinization ? What first follows 

 that stage? What next? What is the final result? What is the whole 

 process called? 



What is seen on watching a drop of fresh drawn blood with the 

 aid of a good microscope? What are the separated threads composed 

 of? To what may we compare a drop of blood in the first formation 

 of the fibrin threads? What do the threads do after their for- 

 mation? 



