CH. IX.] THE COAGULATION OF BLOOD. 237 



serum). Place both tubes in the warm bath at 40 C. and examine 

 from time to time. Clotting takes place in both tubes, but much 

 more rapidly in A than in B. 



NOTE. Dilution with water decreases the concentration of the magnesium 

 sulphate, so that any fibrin formed by the ferment (which can now act on the 

 fibrinogen) becomes insoluble in this low concentration of salt. 



279. The preparation of fibrinogen. To 20 cc. of the salted 

 plasma add an equal volume of a saturated solution of sodium 

 chloride. A precipitate of fibrinogen is formed. Allow the tube to 

 stand for a few minutes and then filter through a small paper. 

 Scrape the precipitate off the paper and treat it with about 5 cc. of 

 5 per cent, sodium chloride. The fibrinogen dissolves. 



NOTE. If bird's blood be drawn directly into a clean vessel in such a way 

 that contact with the tissues is absolutely avoided, it clots very slowly. This is 

 because the leucocytes are very stable and do not liberate thrombokinase. If 

 this blood be centrifugalised at once, a non-clotting plasma is obtained. 

 Fibrinogen can readily be prepared from this by the method given in Ex. 33. 

 The suspension so obtained is dissolved in dilute salt solution. 



280. Divide the solution thus obtained into two portions, C 

 and D. To C add two drops of fibrin ferment. Place both tubes 

 in the warm bath and observe them at intervals. C clots rapidly ; 

 D very slowly. 



Preparation of oxalate plasma. Blood is drawn as in the prepara- 

 tion of salted plasma into a bottle which has 200 cc. of a i per cent, solution 

 of potassium oxalate in place of the 400 cc. of saturated magnesium sulphate. 

 The plasma is separated, as before, by centrifugalisation. 



281. The clotting of oxalate plasma. Dilute 5 cc. of the 

 plasma with 10 cc. of distilled water and divide into three portions, 

 E, F, and G. To E add a few drops of i per cent, calcium chloride ; 

 to F a few drops of fibrin ferment or serum. Place the three tubes 

 on the water bath and observe them at intervals. E clots in a few 

 minutes ; F clots slowly ; G does not clot. 



Preparation of fluoride plasma. This is prepared as oxalate plasma, 

 using a 3 per cent, solution of sodium fluoride in place of the i per cent, 

 potassium oxalate. 



282. The clotting o! fluoride plasma. Dilute 5 cc. with 10 

 cc. of water and divide into three portions/H, K, and L. To H add 

 a few drops of i per cent, calcium chloride ; to K a few drops of fibrin 

 ferment. Place the three tubes in the warm bath and observe them 

 at intervals. K clots rapidly ; H and L do not clot. 



