THE COAGULATION OF THE BLOOD 841 



to clot. On adding a drop of calcium chloride to such plasma we see that it 

 contains an excess of oxalate from the production of a precipitate of calcium 

 oxalate. If calcium chloride be added drop by drop so as to be present in 

 slight excess and the plasma be then kept warm, it will clot within a time 

 varying from a few minutes to an hour. The clot thus formed is perfectly 

 firm and the vessel can be inverted without any of its contents flowing out. 

 Very often no retraction of the clot takes place, but if it be pressed with a 

 glass rod or with the fingers a clear serum is squeezed out and a mass of pure 

 fibrin remains. The place of lime can be taken by strontium, but barium 

 and magnesium are powerless to initiate clotting. We must therefore 

 conclude that the presence of a soluble calcium salt is one factor of those 

 necessary for coagulation. This is borne out by the fact that a similar 

 uncoagulable blood is produced by the action of sodium fluoride. Some 

 difficulty, however, was felt when it was found that sodium citrate might be 

 used instead of sodium oxalate or fluoride, since sodium citrate does not 

 produce in the blood any precipitate of insoluble lime salts. Here therefore 

 we have a blood containing lime in solution and yet uncoagulable. The 

 difficulty has been cleared up by the work of Sabbatani and C. J. Martin. 

 When sodium citrate is added to a lime salt a double salt of sodium calcium 

 citrate is formed in which the calcium is in the anion and forms part of the 

 acidic radical. The mere presence of dissolved calcium is not sufficient 

 for clotting to take place. It is necessary that the calcium should be in the 

 form of a salt and in an ionised condition, such as calcium chloride or calcium 

 sulphate. 



Though calcium is a necessary condition for the occurrence of coagulation, 

 it cannot be regarded as a precursor of the protein fibrin. If the composition 

 of the plasma before coagulation has been set up be compared with that of the 

 serum which has separated from the clot, it is found that plasma contains 

 a protein, fibrinogen, not represented in the serum, which must therefore 

 be the precursor of fibrin. Fibrinogen belongs to the class of globulins. It 

 can be separated from oxalate plasma by half -saturation with common salt. 

 An equal volume of a saturated solution of sodium chloride is added to plasma 

 so that the whole mixture contains 16 per cent, sodium chloride. The fluid 

 gradually becomes turbid from the production of a precipitate which, at 

 fiist granular, rapidly aggregates to form a stringy, slimy solid, and on stirring 

 aggregates into masses which adhere to the glass rod used for stirring. This 

 mass can be taken out of the fluid, washed by kneading with half -saturated 

 sodium chloride solution, and then redissolved by the addition of distilled 

 water. With the salt adhering to the precipitate a dilute saline fluid is 

 formed in which the fibrinogen is soluble. The fibrinogen can be purified by 

 repeating the precipitation and solution, though it tends to lose its solubility 

 in the process, so that purification is always attended with some loss. The 

 solution of fibrinogen thus obtained is perfectly clear and colourless. On 

 warming, practically the whole of its protein is thrown down between 56 

 and 60 C. The same precipitate is produced on heating the original plasma, 

 whereas serum obtained by the expression of the clot does not give any 



27* 



