34 A MANUAL OF PHYSIOLOGY 



cium ions. The following experiments illustrate the role of these 

 three factors : 



The plasma obtained by drawing off bird's blood e.g., the 

 blood of a fowl or goose through a perfectly clean cannula into 

 a perfectly clean vessel, without contact with the tissues, and 

 then rapidly centrifugalizing off the formed elements, can be 

 kept unclotted for days and even weeks. The addition of a 

 small amount of tissue extract (procured by rubbing up blood- 

 free liver, thymus, muscle, or other organs with sand, and ex- 

 tracting for several hours with salt solution) to the bird's plasma 

 causes rapid coagulation. The plasma contains thrombogen 

 and calcium salts, but is lacking in thrombokinase, which is 

 supplied by the tissue extract. A solution of fibrinogen con- 

 taining calcium will clot if serum, in which fibrin-ferment is always 

 present, be added. It will not clot on addition of tissue extract 

 alone, nor on addition of bird's plasma alone (obtained as above), 

 but will readily coagulate if both tissue extract and bird's 

 plasma be added. Therefore, something in the bird's plasma 

 (thrombogen), plus something in the tissue extract (thrombo- 

 kinase), produce in the presence of calcium the same effect as 

 the thrombin of serum. It can be shown that calcium is only 

 necessary for the formation of the fibrin-ferment, 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 0*2 or 

 0-3 per cent., the blood remains unclotted. The plasma separated 

 from this oxalated blood contains both thrombogen and throm- 

 bokinase, but it does not coagulate, because the calcium has 

 been precipitated out in the form of insoluble 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 fibrin- 

 ferment. In the curious hereditary disease known as haemo- 

 philia, a deficiency of calcium seems occasionally to be responsible 

 for the diminished coagulability of the blood ; and the internal 



