CAUSES OF COAGULATION. 51 



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

 be spread out and watched with a microscope magnifying 

 600 or 700 diameters, it will be seen that the 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 a proteid 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 

 tiireads 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 b*e pulled awayj 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.pale corpuscles, 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 certain 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 vig- 

 orously with a bunch of twigs, and the sticky fibrin threads 

 as they form adhere to these. If the twigs be withdrawn 

 after a few minutes a quantity of stringy material will be 

 found attached to them. This is at first colored red by 

 adhering blood corpuscles: but by washing in water these 



