CLOTTING OF PLASMA 305 



Arthus and Pages in 1890 showed the need of calcium in the 

 clotting process. 



Haniniersten confirmed this, and found that the calcium acts 

 on the prothrombin producing active thrombin. 



This still does not explain why blood does not clot in the vessels, 

 so physiologists had to postulate the presence of some substance 

 in the blood which would prevent the calcium from activating 

 the prothrombin. This hypothetical substance they called anti- 

 prothrombin. When a vessel is ruptured and blood comes into 

 contact with the tissues or into contact with disintegrating blood- 

 cells, it takes from them a substance which neutralises the inhibi- 

 tory action of the anti-prothrombin and so allows the calcium to 

 act. This substance was, at that time, believed to be a ferment, 

 or kinase, and was, therefore, named " thrombokinase.'' That is, 

 in all cells there is a large or small quantity of thrombokinase, 

 which is the trigger setting off the whole clotting process. It is 

 now known that this substance is not a ferment or kinase. Some 

 people, therefore, prefer to call it thrombojjiastin. 



That is, we have in the blood-stream prothrombin, anti-pro- 

 thrombin, fibrinogen and calcium salts. In order to start the pro- 

 cess of clotting, thrombin must be liberated from its precursor. 

 This duty falls on thromboplastin, a substance present in all tissues 

 and, in vertebrates, also in the white corpuscles, and perhaps in 

 the platelets {q.v.). Can we form a picture of the process ? 

 Examination of plasma during the process of clotting on a micro- 

 scope slide with dark-ground illumination shows how, on the 

 introduction of a drop of serum, tissue juice, etc., fine needles of 

 fibrin seem to radiate /rom the added fluid. These liquid crystals 

 pack together to form a felt-work of fibrils of fibrin — a very similar 

 process to that studied in an earlier chapter, viz. myelin forms of 

 lecithin (p. 109). 



Pickering and Hewitt have produced evidence to show the 

 lines on which the clotting process runs. Their scheme may have 

 to be modified in detail as our knowledge of the physical chemistry 

 of colloidal systems like blood increases, but it gives a plausible 

 explanation of all the known facts. According to their theory, 

 blood contains all the elements necessary to form a clot, but it 

 maintains its fluidity because of the presence of a protective 

 substance. That is, we have two substances — a sol and a solution, 

 fibrinogen and thrombin respectively, which would interact to 

 form a liquid-crystal complex — a gel, but for the presence of an 

 inhibiting body. This inhibiting body may be a substance acting 

 in a manner similar to the protective emulsoids (Chap. VIII.), or 

 it may interfere with the process of gelation like certain solutes 



B. 20 



