454 BLOOD AND LYMPH. 



like solutions of other proteins, in a great variety of ways, by 

 heat, by acids, by neutral salts, etc., but in these cases the col- 

 loidal particles of the solution are simply aggregated into larger 

 clumps or masses. The deposition of the fibrinogen in the form 

 of needles takes place only under the influence of thrombin, the 

 substance which causes normal clotting, as will be described 

 further on. When the clot is formed it shrinks or contracts 

 rapidly in all directions, especially if freed from contact with the 

 walls of the containing vessel. If the blood has not been dis- 

 turbed during the act of clotting, the red corpuscles are caught 

 in 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 dog's 

 blood 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 focal 

 plane are seen distinctly. 



being squeezed out, so that it is possible to get specimens of 

 serum containing few or no red corpuscles. The leucocytes, on 

 the contrary, although they are also caught at first in the forming 

 meshwork of fibrin, may readily pass out into the serum in the later 

 stages of clotting, on account of their power of making ameboid 

 movements. If the blood has been agitated during the process of 

 clotting, the delicate mesh will be broken in places and the serum 

 will be more or less bloody that is, it will contain numerous red 

 corpuscles. If during the time of clotting the blood is vigorously 

 whipped with a bundle of fine rods, all the fibrin is deposited as a 

 stringy mass upon the whip, and the remaining liquid part con- 



