COAGULATION 37 



for the formation of the thrombin, but not for its action on fibrinogen. 

 For instance, a calcium-free solution of fibrinogen can be made to clot 

 by serum from which the calcium has been removed. 



If a soluble oxalate (potassium or ammonium oxalate) is mixed with 

 freshly drawn dog's blood to the amount of o' 2 or o' 3 per cent., the blood 

 remains unclotted. The plasma separated from this oxalated blood 

 contains both thrombogen and thrombokinase, but it does not coagu- 

 late, because the calcium has been precipitated out in the form of in- 

 soluble calcium oxalate. In the absence of calcium the reaction of the 

 thrombogen and thrombokinase which leads to the formation of 

 thrombin does not take place. All that is necessary to bring about 

 coagulation is to add calcium chloride in somewhat greater quantity 

 than is required to combine with any excess of oxalate present. If more 

 than a certain amount of calcium be added, clotting is hindered instead 

 of being helped, so that it is only within certain limits of concentration 

 that calcium favours coagulation. From oxalate plasma a nucleo- 

 protein or a mixture of nucleo-proteins can be separated which contains 

 thrombogen and thrombokinase, but little or no. calcium, and does not 

 cause clotting, but which on treatment with a calcium salt acquires the 

 properties of thrombin. 



When sodium fluoride is added to freshly drawn blood to the amount 

 of o'3 per cent., coagulation is also prevented. But there is this differ- 

 ence between oxalate and fluoride plasma that, although the calcium 

 has been precipitated in both, the addition of calcium chloride to fluoride 

 plasma is not sufficient to induce clotting. Tissue extract containing 

 thrombokinase must be supplied as well. In some way or other sodium 

 fluoride interferes with the liberation of thrombokinase from the formed 

 elements of the blood, although in the concentration mentioned it does 

 not hinder the action of fully formed thrombin, as is shown by the fact 

 that fluoride plasma coagulates on the addition of a little serum, which 

 supplies thrombin. The fluoride blood clots readily if it is diluted with 

 water, and at the same time mixed with calcium chloride solution, for 

 the water damages the formed elements, and thus favours the liberation 

 of thrombokinase. 



Sodium citrate solution prevents the coagulation of blood run into 

 it, although there is no precipitation of the calcium. The addition of 

 calcium chloride to citrate plasma induces clotting, and the action of 

 the citrate is assumed to be due to the formation of a compound with 

 the calcium of the blood, which does not dissociate so as to yield calcium 

 ions. It ought to be remarked, however, that in all so-called decalci- 

 fied plasmas, as ordinarily obtained, blood-platelets are present, and 

 that platelets disintegrate under the influence of calcium salts. It 

 has been shown, indeed, that many of the reagents and procedures 

 which hinder the clotting of shed blood also prevent the breaking up of 

 the platelets. Thus, the cooling of the blood, the addition of hirudin, 

 sodium oxalate, sodium citrate, manganese salts, etc., which are 

 classical methods used in obtaining platelets for microscopical study, 

 are also classical methods of hindering coagulation. These facts have 

 not hitherto been sufficiently taken account of in interpreting experi- 

 ments on decalcified blood. They indicate that the decalcifying agents 

 may hinder clotting by interfering with the liberation of essential sub- 

 stances from the platelets, and that this may be the decisive factor, and 

 not merely the withdrawal of the calcium from the field where the 

 already liberated thrombokinase and thrombogen would otherwise 

 react to form thrombin. 



When proteoses (or peptones) are injected into the circulation of a 

 dog or goose, the blood is deprived of the power of coagulation. The 



