CH. IX.] THE COAGULATION OF BLOOD. 235 



5. Thrombin, a ferment formed by the interaction 

 of 2, 3 and 4. It probably splits fibrinogen into serum- 

 globulin and fibrin. The latter, being insoluble in the 

 constituents of normal plasma, comes out of solution and 

 with the corpuscles forms the clot. 



The scheme on the following page represents the inter- 

 action of the above factors. 

 Coagulation is hindered by 



1. Cooling. 



2. Substances which precipitate calcium salts, or 

 convert the calcium into the non-ionised condition, as 

 oxalates, citrates and soap solutions. 



3. Alkalies, which prevent the liberation of throm- 

 bokinase by the corpuscles, delay the action of thrombin, 

 and tend to dissolve fibrin. 



4. Strong salt solutions, which have a similar action. 



5. Anti-thrombin, a substance found in small amounts 

 in the plasma, and in relatively large amounts in extracts 

 of the head of the leech. It combines with thrombin to 

 render it inactive. 



6. Anti-kinase, found in the blood, after the slow 

 injection into the blood stream of certain substances, as 

 tissue-extracts, certain snake-venoms, etc. 



7. Fluorides, which precipitate calcium salts and pre- 

 vent the liberation of thrombokinase. 



Preparation of fibrin ferment (thrombin). Blood serum is treated with 

 four or five times its volume of strong alcohol, well stirred and allowed to 

 stand for two or three days. The precipitate is collected, dried on filter paper 

 in the air, and extracted with water. The filtered extract contains fibrin 

 ferment. 



Preparation of "salted" plasma. Two litres of water are placed in a 

 large bottle or jar (provided with a tightly-fitting stopper) and the level of 

 the fluid marked by a label. The water is poured off and 400 cc. of a saturated 

 solution of magnesium sulphate substituted. Blood is collected in the bottle 

 till the level is reached, care being taken to ensure thorough mixing with the 

 salt solution by stopping the flow of blood from time to time and turning the 

 bottle upside down. The corpuscles are removed by centrifugalisation and 

 the plasma pipetted off. It should be kept in a refrigerator till required. 



278. The clotting of salted plasma. Take 2 cc. of salted 

 plasma in a test-tube, add 10 cc. of water, and divide into two 

 portions, A and B. To A add a few drops of fibrin ferment (or of 



