1 66 THE BLOOD. 
substance which possesses the property of converting fibrinogen into 
fibrin, and is, according to Pekelharing, a combination of the nucleo- 
proteid with lime, and identical with the fibrin ferment of A. Schmidt. 
The fibrin ferment is sometimes spoken of as "thrombin," and the 
nucleo-proteid material in the plasma from which it is produced is then 
termed " prothrombin." 
Wooldridge 1 found that, on subjecting peptone plasma to cold, he 
obtained a finely granular deposit, which had the property of producing 
clotting in fibrinogenous fluids, which are not themselves spontaneously 
coagulable, and of accelerating the process of clotting in coagulable fluids. 
To the material thus obtained, and which he described as having, under 
the microscope, an appearance similar to masses of blood platelets, he 
gave the name " A-fibrinogen," because he found that on adding it to 
peptone plasma it produced fibrin, and that the amount of coagulation 
was more or less proportional to the amount of A-fibrinogen added. It 
is not fibrinogen as the term is ordinarily used, but is probably either a 
nucleo-proteid, or a mixture of nucleo-proteid with globulin. A similar 
deposit occurs, as already stated, in oxalate plasma, on standing in the 
cold. A precipitate containing the same substance is also produced by 
adding magnesium sulphate solution in considerable amount to blood, and 
in both plasma and serum of certain animals on acidulation with acetic 
acid, but in both cases it is liable to be mixed with serum globulin. 
It also occasionally occurs in serum, on standing, even without the 
application of cold. Halliburton has suggested that the deposit in 
peptone plasma may be a part of the proteoses, which were injected into 
the blood, for he found that solutions of albumose were liable to give a 
similar deposit on cooling by means of ice, but there is not enough proteose 
present in peptone plasma to account for such deposit, and the fact that it 
occurs under other conditions in plasma also negatives this supposition. 
These experiments of Wooldridge, and the behaviour of the body termed 
by him A-fibrinogen, will be again referred to in a subsequent section. 
Fibrin. — Fibrin is the chief substance formed from fibrinogen in the 
coagulation of blood plasma, and it is also produced in the coagulation of 
lymph and other fibrinogen-containing fluids. It is usually got 1 >y whipping 
blood as it flows from the blood vessels with a bundle of wires or glass 
rods before it has had time to coagulate into a solid mass. The coagulum 
then forms upon the wires or rods, and can be washed free from adherent 
red corpuscles by putting it under a stream of water for a few hours. 
But to obtain pure fibrin it is necessary first to prepare fibrinogen from 
blood plasma by precipitation with NaCl (half-saturated), to purify this 
by re-solution and re-precipitation, and finally to cause the coagulation of 
the fibrinogen solution by fibrin ferment. The clot thus obtained, which 
must be thoroughly washed, is composed of nearly pure fibrin. 
When obtained by whipping blood, fibrin is a white stringy substance 
when wet, drying to a glue-like mass. The threads of which it is com- 
posed, and which, as may be seen in a microscopic preparation of blood, 
interlace with one another and form a network of the finest possible 
filaments, entangling the blood corpuscles in its meshes, have a strong- 
tendency to retract or shorten when formed ; this is the reason why a 
clot shrinks and expresses serum from its interior. Fibrin is slowly 
soluble in 5 to 10 per cent, solutions of certain salts, such as sodium 
chloride, sodium sulphate, potassium nitrate, magnesium sulphate, and 
1 Wright, Lancet, London, 1892, vol. i. pp. 457, 515. 
