842 PHYSIOLOGY 



precipitate on heating until a temperature of 68 to 70 C. is reached. If a 

 solution of fibrinogen, obtained by precipitating with sodium chloride and 

 dissolving in distilled water, be treated with a drop or two of calcium chloride 

 it rapidly clots with the formation of typical fibrin. We might conclude 

 from this experiment that fibrin was a compound produced by the union 

 of calcium salts with fibrinogen. Further experiments show, however, the 

 untenability of this hypothesis. If the fibrinogen has been thoroughly 

 purified by repeated precipitation and re-solution, calcium salts are found 

 to have entirely lost their power of causing coagulation. Such a purified 

 fibrinogen can still be made to clot by the addition either of serum or of the 

 washings of a blood-clot, or of the watery extract of alcohol-coagulated 

 serum. This power of serum to convert fibrinogen into fibrin is due to the 

 presence in it of minute quantities of a substance which has been designated 

 as ' fibrin ferment ' or thrombin. It has been regarded as a ferment because 

 it is active in minimal quantities and is stated not to be appreciably used up 

 in the process of clotting. Thus if we add some serum to a fibrinogen 

 solution we can cause clotting, and then on squeezing the clot obtain a serum 

 which will bring about coagulation when added to a fresh portion of fibrino- 

 gen. Considerable doubt, however, has been thrown on the ferment nature of 

 thrombin by the recent work of Rettger in Howell's laboratory. Rettger 

 shows, in the first place, that the statement of the preceding sentence is only 

 true if a large excess of thrombin solution be made use of in the first instance. 

 If small quantities of thrombin solution be added to large quantities of 

 plasma or of pure solutions of fibrinogen, the amount of fibrin obtained 

 is proportional to the amount of thrombin added. This is shown in the 

 following experiment : 



5 drops of thrombin gave 0-2046 grm. fibrin. 

 10 0-3573 



20 0-6089 



40 1-5872 



Moreover the action of thrombin on fibrinogen solutions is almost 

 independent of temperature, occurring practically as rapidly at 17 C. as 

 at 40 C. It has been found that there is only a slight increase in the rate 

 of action of snake venom on fibrinogen solutions on warming from 20 to 

 40 C. Rettger therefore regards fibrin as formed by the union of thrombin 

 and fibrinogen. This union is apparently very unstable. If fibrin be 

 extracted with a 3 per cent, salt solution for some time, part of it goes into 

 solution, and the solution is found to contain thrombin. In the same way the 

 thrombin may be re-extracted from the fibrin if the latter be allowed to 

 putrefy. 



From all the different kinds of plasma which have been enumerated above 

 the purified fibrinogen can be obtained by the use of sodium chloride, and 

 in every case can be made to clot by the addition of serum or of a solution of 

 thrombin. The last change in the act of clotting is therefore the change from 

 fibrinogen to fibrin, and this event is brought about by the intervention of 



