We now come to the consideration of the natural modifications of protein, which we find composing the chief bulk of the bodies of animals, viz. fibrin, albumen, and ca.sein.
Fibrin. — This is a substance of the highest importance in the animal economy, since it is the material of which the solid framework of the muscles and some other tissues mainly consist ; and it is also found dissolved in the blood, from which it separates spontaneously after removal from the body, forming the clot or crassamentum. The following table shows the average proportion of fibrin in several animal products.
Fibrin may be obtained from lean animal flesh by cutting it into thin slices and washing with water till it is colourless ; it is, however, impossible to obtain it pure in this way, as it is always associated with fatty matters. nerves.
and membrane. It may be obtained in a state of purity from the blood, in which, as already mentioned, it exists in a soluble condition, but remarkably- prone to assume the solid form as soon as removed from the body. The blood, as soon as drawn, should be rapidly beaten up with a bundle of wires or twigs, to which the fibrin attaches itself in the form of solid amor phous filaments, coloured red by a quantity of the globules entangled in its pores during the coagulation ; these latter may be removed by placing the coagulum in a piece of linen cloth, and washing with a stream of cold water until all colour disappears. It still contains fatty matters, inorganic salts, and a considerable quantity of water, all which may be removed by drying on a chloride of calcium bath at a temperature of about 230°, pounding the hard mass in a mortar, washing with alcohol, ether, and dilute hydrochloric acid, and lastly, mace rating in water until all soluble matter is dis solved out, when it should be again thoroughly dried. Thus prepared, it is of a yellowish colour, hard, brittle, and, when perfectly free from fat, transparent. It is tasteless, and inso luble in alcohol, ether, and water ; but in the latter it softens, swells up, and reassumes the appearance it had previous to desiccation. Though insoluble in both hot and cold water, it is converted by prolonged boiling, first into binoxide and eventually into tritoxide of protein, which latter is soluble in water.
Most of the acids, when in a concentrated state, cause fibrin to swell up and assume a gela tinous appearance. It was observed by Scherer that when moist fibrin is placed in an atmo sphere of oxygen, it has the property of ab sorbing and retaining a portion of the gas ; an effect no doubt accompanied by the formation of one or more of the oxides of protein : it is probable that a portion of the fibrin of the blood undergoes a similar change, since these oxides are always present in arterial blood both in health and disease, especially in some forms of fever, when, by an accelerated respiration, a larger amount of oxygen is introduced into the system.
Fibrin and sulphuric acid. —With strong sul phuric acid dry fibrin becomes yellowish and gelatinous, considerable heat being at the same time evolved, sufficient indeed, provided the quantity be large, to cause complete decompo sition, when it blackens, and sulphurous acid is given off. When water is added, the gelatinous mass contracts suddenly in bulk, and the white curdy matter thus obtained consists chiefly of sulphoproteic acid, already described.
Fibrin and nitric acid. — Fibrin behaves with nitric acid in a similar manner to protein, giving rise to the formation of xanthoproteic acid.
Fibrin and acetic acid. —When treated with concentrated acetic acid, it almost immediately becomes gelatinous, and if water be added and the mixture warmed, it readily dissolves, espe cially if the fibrin be obtained from a young animal : this solution when evaporated leaves the fibrin with precisely' the same properties which it had previous to dissolution. .If an other acid, as the sulphuric, be added to the acetic solution, it combines with the protein, forming generally an insoluble compound, as in the case of the sulphobiproteic acid. If the acetic acid solution be neutralized with potash, the fibrin is precipitated, but is redissolved if the alkali be added in excess.