222 THE CIRCULATORY APPARATUS 



material to the blood plasma, which aids in the 

 coagulation of blood. 



Blood Plaques. These are colorless disks con- 

 sisting of protoplasm. Their diameter is 1.5 to 3.5 

 micromillimeters. The number compared to the red 

 cells is 1 to 18 or 20. They are concerned mostly 

 with the coagulation of the blood, by their adhering 

 and forming irregular masses (Schultze), acting as a 

 nucleus for the fibrin filaments to spread from during 

 coagulation of the blood. They can only be seen 

 microscopically after subjecting the blood to treat- 

 ment with osmic acid. 



Coagulation of Blood. Blood when freshly drawn from 

 a living body into a vessel is fluid. In a short time 

 it becomes thickened or viscid, this increase in con- 

 sistency becomes more marked .until the vessel con- 

 tains a dark reddish mass, resembling gelatin. Shortly 

 a few drops of fluid appear on the surface of the mass, 

 which gradually increases in amount, the vessel is 

 seen to contain a deposit of a firm, organized mass 

 the clot floating in a reddish-yellow fluid the 

 blood-serum. On examining a portion of the clot 

 microscopically, it will show threads of fibrin with 

 red and white corpuscles clinging to them. 



The Clotting of Blood. This is supposed to be a 

 chemic phenomenon due to the action of a ferment, 

 derived from calcium chloride, and some authors 

 suggest leukocytes acting on the fibrinogen of the 

 blood plasma, and converting it into fibrin and thus 

 forming the nucleus of the clot. If blood is freshly 

 drawn into a vessel, then whipped with a bundle of 

 fine twigs for a few moments, the fibrin will be deposited 

 on these twigs as whitish threads. Blood treated in 

 this manner will not clot when left in the vessel; the 

 serum will be the only residue present. This blood, 

 treated as above, is called defibrinated. 



