THE INDIVIDUAL GROUPS OF PROTEINS 441 



The freshly prepared threads of fibrin are nearly colourless ; on 

 drying by exposure to the air they form a brownish mass. The fresh 

 threads are best preserved in glycerin, which is easily removed by 

 washing. 



Fibrin is insoluble in water, salt solutions, cold dilute solutions of 

 acids and alkalies. It dissolves on warming in dilute acid or alkali, 

 but undergoes hydrolysis into its derivatives. 



A suspension of fibrin in water will give the colour reactions for 

 proteins. The solution in acid or alkali will behave like metaprotein, 

 or proteoses and peptone, depending on the length of time the solu- 

 tion has been heated. It will be precipitated by heavy metals and 

 alkaloidal reagents. 



Thrombin. 



Thrombin is not present in blood plasma, but is formed in the process of 

 clotting ; it will therefore be present in the serum and upon the fibrin. 



(1) Preparation from Serum or Defibrinated Blood. 



i volume of serum or defibrinated blood is mixed with 15-20 volumes 

 of alcohol and the mixture is allowed to stand for some weeks. The pre- 

 cipitate, which is formed, is filtered off, washed with alcohol and dried in 

 a desiccator. It contains thrombin, which is extracted by water (Schmidt). 



(2) Preparation from Fibrin. 



It is best to use fibrin which has been obtained from blood diluted with 

 10 volumes of water and which has been washed with water. This fibrin is 

 preserved in weak alcohol. A solution of thrombin is obtained by extracting 

 the fibrin with 8 per cent, sodium chloride solution (Gamgee). 



IV. The Formation of Fibrin by Interaction of Calcium Salts, 

 Thrombogen and Thrombokinase. 



The formation of fibrin is easily observed by the following experi- 

 ments with the salt plasmas : 



(a) Sodium Sulphate Plasma. 



Clotting occurs on diluting a small quantity with 5 volumes of 

 water and keeping the solution warm at 40. The clotting is more 

 rapid if a drop of serum or thrombin solution be added. 



This plasma contains fibrinogen and also thrombin. They have 

 been prevented from interacting by the presence of neutral salts, hence 

 on dilution the plasma clots. 



(b) Magnesium Sulphate Plasma. 



This plasma on dilution with 9 volumes of water and warming 

 to 40 does not clot. 



It clots on diluting as above and adding a drop of serum or throm- 

 bin solution. 



This plasma contains fibrinogen, but no thrombin. The formation 

 of thrombin has been prevented by the presence of magnesium sul- 

 phate. 



