DISEASE-RESISTING FUNCTIONS OF THE BLOOD 313 



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 

 away, and thus the first sign of the contraction of the fibrin is 

 seen in the cupping of the surface of the gelatinized blood where 

 the threads have no solid attachment, and there the contracting 

 mass presses out from its meshes the first drops of serum. Finally 

 the contraction of the fibrin overcomes its adhesion to the vessel 

 and the clot pulls itself loose on all sides, pressing out more and 

 more serum, in which it ultimately floats. The great majority 

 of the red corpuscles are held back in the meshes of the fibrin, 

 but a good many leucocytes, by their amoeboid movements, 

 work their way out and get into the serum. 



Whipped Blood. The essential point in coagulation being the 

 formation of fibrin in the plasma, and blood only forming a cer- 

 tain amount of fibrin, if this be removed as fast as it forms the 

 remaining blood will not clot. The fibrin may be separated by 

 what is known as " whipping" the blood. For this purpose fresh- 

 drawn blood is stirred up vigorously with a bunch of twigs, and 

 to these the sticky fibrin threads as they form, adhere. If the 

 twigs be withdrawn after a few minutes a quantity of stringy 



