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.