34 THE CIRCULATING LIQUIDS OF THE BODY 



blood may be used to delay coagulation, the blood being run direct 

 from the animal into, say, a third of its volume of saturated mag- 

 nesium sulphate solution. The plasma may then be conveniently 

 separated from the corpuscles by means of a centrifugal machine. 

 Again, two ligatures may be placed on a large bloodvessel, so that 

 a portion of it can be excised full of blood and suspended vertically 

 (the so-called experiment of the ' living test-tube ') ; coagulation 

 is long delayed, and the corpuscles sink to the lower end. In these 

 and many other ways plasma free from corpuscles can be got ; and 

 it is found that when the conditions which restrain coagulation are 

 removed when, for instance, the temperature of the horse's plasma 

 is allowed to rise or the magnesium sulphate plasma is diluted with 

 several times its bulk of water clotting takes place, with forma- 

 tion of fibrin in all respects similar to that of ordinary blood-clot. 

 The corpuscles themselves cannot form a clot.* From this we con- 

 clude that the essential process in coagulation of the blood is the 

 formation of fibrin from some constituent of the plasma, and that 

 the presence of corpuscles in ordinary blood-clot is accidental. 

 In accordance with this conclusion, we find that lymph entirely 

 free from red corpuscles clots spontaneously, with formation of 

 fibrin ; and when fibrin is removed from newly shed blood by whip- 

 ping it with a bundle of twigs or a piece of wood, it will no longer 

 coagulate, although all the corpuscles are still there. 



What, now, is the substance in the plasma which is changed into 

 fibrin when blood coagulates ? If plasma, obtained in any of the 

 ways described, be saturated with sodium chloride, a precipitate is 

 thrown down. The filtrate separated from this precipitate does not 

 coagulate on dilution with water; but the precipitate itself the 

 so-called plasmine of Denis on being 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) . Hammersten, 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 coagu- 

 lates; and a solution of pure fibrinogen can be made to clot and 

 to form fibrin, while a solution of serum-globulin cannot. Blood- 



* Bird's corpuscles, however, washed free from plasma, will form a clot 

 when laked in various ways, as by addition of water or by freezing and 

 thawing. 



