THE FLUIDS OF THE BODY 75 



light which warn the physiologist that as yet his theory of 

 coagulation is incomplete. 



The presence of salts of lime has an important relation to 

 coagulation. If blood is received into a vessel in which has 

 been placed some powdered oxalate of potash, or soap, or any 

 other chemical which fixes lime, the blood does not coagulate. 

 All other conditions are as usual, but lime is withdrawn from 

 the plasma. The non-coagulation of oxalated plasma was 

 interpreted as indicating that lime, under the influence of 

 fibrin -ferment, combines with fibrinogen to form fibrin ; that 

 fibrinogen altered by fibrin-ferment combines with lime. This 

 hypothesis was based upon the analogy of the curdling of 

 milk. Milk cannot curdle if lime be absent. If rennin (milk- 

 ferment), prepared from milk from which lime has been re- 

 moved, be added to a solution of caseinogen (the coagulable 

 protein of milk), also prepared from lime-free milk, no curd is 

 produced. The addition of a few drops of a solution of chloride 

 of lime results in the immediate curdling of the mixture. 

 Evidently rennin so alters caseinogen as to bring it into a 

 condition to combine with lime. But the analogy does not 

 hold good for blood. In the case of plasma, lime acts, not upon 

 fibrinogen, but upon the fibrin-ferment or rather upon a 

 precursor of fibrin-ferment in such a way as to render it 

 effective. Leucocytes produce a prothrombin, which in contact 

 with lime-salts is converted into thrombin, which coagulates 

 fibrinogen. 



Fibrinogen is the substance which fibrin-ferment combined 

 with salts of lime changes into fibrin. Yet even now the 

 story is not complete, if the theory of coagulation is to be 

 brought up to date. A perfectly clean cannula is passed into 

 an artery of a bird. If it be thrust well beyond the place 

 where the vessel has been cut, if the vessel be tied so gently as 

 to avoid injury to its inner coat, and if the blood which first 

 passes through the cannula be allowed to escape, the blood 

 subsequently collected will not clot. It contains fibrinogen, 

 lime salts, and fibrin-ferment, ordinarily so called ; but the 

 ferment is ineffective. The addition to the blood of a frag- 

 ment of injured tissue, or of a watery extract of almost any 

 tissue, immediately sets up coagulation. This observation 

 brings fibrin-ferment into line with other ferments. Digestive 



