Fibrin is a proteid substance which appears in the blood after it is shed, and by its appearance gives rise to the process of coagulation or clotting. Freshly prepared fibrin is a stringy, elastic, white substance. Its elementary composition is carbon, 52.6; hydrogen, 7.0; nitrogen, 17.4; sulphur, 1.2; oxygen, 21.8 = 100.0. It gives the various reactions of the proteid substances (see PROTEIDS). It is insoluble in water, and only slowly soluble in solutions of the neutral salts. It is precipitated by a saturated solution of sulphate of magnesium. It must therefore be considered as nearly allied to the globulin group of proteids.
Fibrin may best be obtained by whipping blood as it is shed with a bundle of twigs, which after a time become surrounded by masses of the fibrous elastic threads of fibrin. These may then be washed to free them from the various constituents of the blood. The mode of appearance may be well studied by allowing a layer of blood to coagulate on a microscopic slide under a cover glass, and then gently washing the cover glass to which the clot adheres with a stream of water. It will be seen to consist of a delicate reticulum of fibrils with granular-looking masses at many of the nodal points. The fibrils appear to have shot out from these granular masses.
The source of fibrin is a matter upon which our knowledge is at present imperfect. The most recent investigations tend to show that a substance belonging to the globulin group of proteids, and known as fibrinogen, which occurs in the blood- plasma before coagulation, becomes precipitated as fibrin when the blood is shed. What is the cause of this precipitation we do not know. Formerly it was supposed that the white corpuscles broke down and set free a ferment which set up the change. More recently the possible connection of the third element of the blood—the blood platelets—with the process has been suggested by various pathological investigations. Apparently the granular nodal masses already described are composed of these platelets. Although the wet fibrin derived from blood appears somewhat bulky, when dried and weighed its amount is found to be very small—on an average only 0·2 per cent. See BLOOD.