280 THE HUMAN BODY 



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 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 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 twigs be withdrawn after a few minutes a quantity of stringy 

 material will be found attached to them. This is at first colored 

 red by adhering blood-corpuscles : but by washing in water they 

 may be removed, and the pure fibrin thus obtained is perfectly 

 white and in the form of highly elastic threads. It is insoluble in 

 water and in dilute acids, but swells up to a transparent jelly in 

 the latter. The " whipped " or " defibrinated blood " from which 

 the fibrin has been in this way removed, looks just like ordinary 

 blood, but has lost the power of coagulating spontaneously. 



The Buffy Coat. That the red corpuscles are not an essential 

 part of the clot, but are merely mechanically caught up in it, 

 seems clear from the microscopic observation of the process of 

 coagulation; and from the fact that perfectly formed fibrin can 

 be obtained free from corpuscles by whipping the blood and 

 washing the threads which adhere to the twigs. Under certain 

 conditions, moreover, one gets a naturally formed clot containing 

 no red corpuscles in one part of it. The corpuscles of human blood 

 are a little heavier, bulk for bulk, than the plasma in which they 

 float; hence, when the blood is drawn and left at rest they sink 

 slowly in it ; and if for any reason clotting take place more slowly 

 or the corpuscles sink more rapidly than usual, a colorless top 

 stratum of plasma, with no red corpuscles in it, is left before 



