. A text-book of physiology : for medical students and physicians . osol, is unique so far asour knowledge goes. Solutions of fibrinogen may be precipitated, * See Stiibel, Pfiugers Archiv f. d. ges. Physiologie, 156, 361, 1914; andHowell, American Journal of Physiology, 35, 143, 1914. COAGULATION. 453 like solutions of other proteins, in a great variety of ways, byheat, by acids, by neutral salts, etc., but in these cases the col-loidal particles of the solution are simply aggregated into largerclumps or masses. The deposition of the fibrinogen in the formof needles takes place only under the
. A text-book of physiology : for medical students and physicians . osol, is unique so far asour knowledge goes. Solutions of fibrinogen may be precipitated, * See Stiibel, Pfiugers Archiv f. d. ges. Physiologie, 156, 361, 1914; andHowell, American Journal of Physiology, 35, 143, 1914. COAGULATION. 453 like solutions of other proteins, in a great variety of ways, byheat, by acids, by neutral salts, etc., but in these cases the col-loidal particles of the solution are simply aggregated into largerclumps or masses. The deposition of the fibrinogen in the formof needles takes place only under the influence of thrombin, thesubstance which causes normal clotting, as will be describedfurther on. When the clot is formed it shrinks or contractsrapidly in all directions, especially if freed from contact with thewalls of the containing vessel. If the blood has not been dis-turbed during the act of clotting, the red corpuscles are caughtin the fine fibrin meshwork, and as the clot shrinks these cor-puscles are held more firmly, only the clear liquid of the blood. Fig. 186.—The fibrin needles formed in the clotting of blood. Plasma of oxalated dogsblood clotted by thrombin. The photographs show the needles as seen with the ultrami-croscope. A, photographed by sun-light; B, by arc-light. Only the needles lying in the focalplane are seen distinctly. being squeezed out, so that it is possible to get specimens ofserum containing few or no red corpuscles. The leucocytes, onthe contrary, although they are also caught at first in the formingmeshwork of fibrin, may readily pass out into the serum in the laterstages of clotting, on account of their power of making ameboidmovements. If the blood has been agitated during the process ofclotting, the delicate mesh will be broken in places and the serumwill be more or less bloody—that is, it will contain numerous redcorpuscles. If during the time of clotting the blood is vigorouslywhipped with a bundle of fine rods, all the fibrin is dep
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