32 A MANUAL OF PHYSIOLOGY 



dissolved in a little water, does form a clot. Fibrin is therefore 

 derived from something in this precipitate. Now, ' plasmine ' 

 contains two protein bodies fibrinogen, which coagulates by 

 heat at about 56 C., and serum-globulin, which coagulates 

 at about 75 C., and it was at one time believed that both of 

 these entered into the formation of fibrin (Schmidt). Hammer- 

 sten, however, has shown that fibrinogen alone is a precursor 

 of fibrin ; pure serum-globulin neither helps nor hinders its 

 formation. This observer isolated fibrinogen from blood- 

 plasma by adding sodium chloride till about 13 per cent, was 

 present. With this amount the fibrinogen is precipitated, 

 while serum-globulin is not precipitated till 20 per cent, of 

 salt is reached. After precipitation of the fibrinogen the plasma 

 no longer coagulates ; and a solution of pure fibrinogen can 

 be made to clot and to form fibrin, while a solution of serum- 

 globulin cannot. Blood-serum, too, which contains abundance 

 of serum-globulin, but no fibrinogen, will not coagulate. 



So far, then, we have reached the conclusion that fibrin is 

 formed by a change in a substance, fibrinogen, which can be 

 obtained by certain methods from blood-plasma. It may be 

 added that there is evidence that fibrinogen exists as such in 

 the circulating blood ; for if unclotted blood be suddenly heated 

 to about 56 C., the temperature of heat-coagulation of fibrinogen, 

 the blood for ever loses its power of clotting. The liver seems 

 to be an important place of origin of fibrinogen, which may also 

 be formed in the bone-marrow. Since fibrinogen is readily 

 soluble in dilute saline solutions, and fibrin only soluble with 

 great difficulty, we may say that in coagulation of the blood a 

 substance soluble in the plasma passes into an insoluble form. 



How is this change determined when blood is shed ? We 

 have said that a solution of pure fibrinogen can be made to 

 coagulate, but it does not coagulate of itself. The addition of 

 another substance in minute quantity is necessary/ This other 

 substance does not itself seem to be used up in the process, nor 

 to enter bodily into the fibrin formed ; a small quantity of it 

 can cause an indefinitely large amount of fibrinogen to clot ;* 

 its power is abolished by boiling. For these reasons it is con- 

 sidered to be a ferment (p. 314), and is spoken of as fibrin- 

 ferment or thrombin. 



Two forms of fibrin-ferment have been distinguished by some 

 writers : a -thrombin, which exists in serum before anything has 

 been added to it, and /3-thrombin, or Schmidt's fibrin-ferment, in 



* According to Rettger's recent work this is erroneous. He states that 

 the quantity of fibrin formed is proportional to the amount of thrombin 

 present, and that thrombin does not act like a ferment. The inquiry is 

 complicated by the fact that fibrin, once formed, tends to adsorb the 

 remaining thrombin and so to interfere with its further action. 



