FIBRIN, or FIBRINE  , a protein formed by the action of the so-called fibrin-ferment on fibrinogen, a constituent of the blood-plasma of all vertebrates . This change takes place when blood leaves the arteries, and the fibrin thus formed occasions the clotting which ensues (see BLOOD) . To obtain pure coagulated fibrin it is best to heat blood-plasma (preferably that of the horse) to 56° C . The usual method of beating a blood-clot with twigs and removing the filamentous fibrin which attaches itself to them yields a very impure product containing haemoglobin and much globulin; moreover, it is very difficult to purify . Fibrin is a very voluminous, tough, strongly elastic, jelly-like substance; when denaturalized by heat, alcohol or salts, it behaves as any other coagulated albumin .