334 PEACTICAL PHYSIOLOGY 



as those given in Experiment III. Sodium citrate, it will be found, 

 acts like oxalate. This is not because it precipitates the calcium, but 

 because it combines with it to form a soluble citrate, a salt which does 

 not ionise (dissociate) in solution, and therefore leaves no calcium free 

 to aid in the formation of thrombin. If fluoride has been used, it will 

 be found that the addition of calcium salts to the plasma does not cause 

 a clot to form, showing that the fluoride in some way prevents the 

 formation of the enzyme from the pro-enzyme. 



We are now in a position to discuss the chief phenomena concerned 

 in the coagulation of blood. We have learnt that a soluble protein, 

 fibrinogen, under the action of an enzyme called thrombin, is turned 

 into an insoluble protein termed fibrin. In the absence of calcium 

 salts blood will not clot, not because this enzyme cannot act, but 

 because it cannot be formed from the pro-enzyme thrombogen in the 

 absence of such salts. It has recently been shown also that the admix- 

 ture of tissue juice plays an important part in the liberation of the 

 enzyme from the pro-enzyme. It is well known that jagged wounds 

 stop bleeding more easily than clean-cut wounds, as also do wounds 

 through a thick piece of skin compared to those through a thin piece. 

 Similarly, the coagulation of bird's blood, which normally clots very 

 slowly if the blood has not come in contact with foreign tissues, can 

 be greatly hastened by the addition of some tissue juice. This body 

 existing in the tissue juice is known as thrombokinase. The pro- 

 enzyme is present in the plasma, as it is precipitated with fibrinogen 

 from bird's plasma on the addition of distilled water. In mammals the 

 thrombokinase is also yielded by the disintegration of the white cor- 

 puscles and the platelets. 1 We can therefore draw up the following 

 table for the coagulation of blood : 



Thrombokinase + Thrombogen + Ca salts. 



(from tissue juice, (plasma) (plasma) 



white corpuscles, 



platelets). 



Thrombin = FIBRINOGEN (soluble) 



I 

 FIBRIN (insoluble). 



(Compare this with the clotting of milk (p. 328), noting the difference 



1 Different authorities hold different views as regards blood platelets. Some 

 regard them as an actual corpuscle. Others look upon them rather as products 

 of asphyxiated blood coming either from the disintegration of the white cor- 

 puscles or the fine coagulation of the blood proteins. 



