FUNCTIONS OF THE BLOOD 279 



from the living Body it is perfectly liquid, flowing in any direction 

 as readily as water. This condition is, however, only temporary; 

 in a few minutes the blood becomes viscid and sticky, and the 

 viscidity becomes more and more marked until, after the lapse 

 of five or six minutes, the whole mass sets into a jelly which ad- 

 heres to the vessel containing it, so that this may be inverted 

 without any blood whatever being spilled. This stage is known as 

 that of gelatinization and is also not permanent. In a few minutes 

 the top of the jelly-like mass will be seen to be hollowed or 

 "cupped" and in the concavity will be seen a small quantity of 

 nearly colorless liquid, the blood-serum. The jelly next shrinks 

 so as to pull itself loose from the sides and bottom of the vessel 

 containing it, and as it shrinks 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 coagulated though retaining its 

 form, floating in a quantity of pale yellow serum. If, however, 

 the blood be not allowed to coagulate in perfect rest, a certain 

 number of red corpuscles will be rubbed out of the clot into the 

 serum and the latter will be more or less reddish. The longer the 

 clot is kept the more serum will be obtained : if the first quantity 

 exuded be decanted off and the clot put aside and protected from 

 evaporation, it will in a short time be found to have shrunk to a 

 smaller size and to have pressed out more serum; and this goes on 

 until putrefactive changes commence. 



Cause of Coagulation. If a drop of fresh-drawn blood be spread 

 out very thin and watched for a few minutes with a microscope 

 magnifying 600 or 700 diameters, it will be seen that the coagu- 

 lation is due to the separation of very fine solid threads which 

 run in every direction through the plasma and form a close net- 

 work entangling all the corpuscles. These threads are composed 

 of the protein substance 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 cavi- 

 ties. After the fibrin threads have been formed they tend to 

 shorten; hence when blood clots in mass in a vessel, the fibrinous 

 network tends to shrink in every direction just as a network 

 formed of stretched india-rubber bands would, and this shrinkage 

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

 stick too firmly to the bottom and sides of the vessel to be pulled 



