450 BLOOD AND LYMPH. 



from the blood-vessels. The general changes in the blood during 

 this process are easily followed. At first perfectly fluid, in a few 

 minutes it becomes viscous and then sets into a soft jelly which 

 quickly becomes firmer, so that the vessel containing it may be 

 inverted without spilling the blood. The clot continues to grow 

 more compact and gradually shrinks in volume, pressing out a 

 smaller or larger quantity of a clear, faintly yellow liquid to which 

 the name blood-serum is given. The essential part of the clot is the 

 fibrin. Fibrin is an insoluble protein not found in normal blood. 

 In shed blood, and under certain conditions in blood while still in the 

 blood-vessels, this fibrin is formed from the soluble fibrinogen. 

 The deposition of the fibrin is peculiar. As seen in ordinary 

 microscopical preparations, the fibrin forms very delicate threads 

 which are united to make a fine reticulum. When the process is 

 observed with the aid of the ultramicroscope it can be seen that 

 the fibrin is deposited in the form of very fine needles, which have 

 the appearance of acicular crystals.* When the process of clot- 

 ting is very slow the needles may remain separate for a long time 

 until their number is greatly increased, but in normal clotting the 

 process, once it has started, is completed quite rapidly, the final 

 result being a dense network or meshwork of the fibrin needles 

 (Fig. 186). This form of precipitation of a colloid solution, or 

 the formation of a hydrogel from a hydrosol, is unique so far as 

 our knowledge goes. Solutions of fibrinogen may be precipitated, 

 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 

 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 



* See Stiibel, "Pfluger's Archiv f. d. ges. Physiologie," 156, 361, 1914; and 

 Howell, "American Journal of Physiology," 35, 143, 1914. 



