THE BLOOD 189 



Fibrin is a proteid substance. It is slowly dissolved in 

 solutions of neutral salts. It is coagulated by heat, and is 

 precipitated when an excess of a neutral salt is added. It 

 is therefore a globulin. 



The plasma before clotting and the serum squeezed out 

 from the clot agree in containing an albumin (serum albumin) 

 and a globulin, or series of globulins which may be classed 

 together as serum globulin, in the same proportion in each. 

 But the plasma contains a small quantity about 0*2 per 

 cent. of another globulin (fibrinogen) which coagulates at a 

 low temperature, and which is absent from serum. It is this 

 which undergoes the change from the soluble form to the 

 insoluble form in coagulation. When separated from the 

 other proteids it can still be made to clot. 



The substance which usually causes clotting appears to be 

 an enzyme, which is formed by the union of a nucleo-proteid 

 or a derivative of a nucleo-proteid with a lime salt. The 

 enzyme may be called thrombin,and its precursor prothrom- 

 bin. Oxalates, when added to blood, precipitate the soluble 

 lime salts, and prevent the formation of thrombin, and thus 

 prevent coagulation. 



There is some evidence that the prothrombin exists in 

 solution in the plasma, but it certainly is derived from the 

 breaking down of the cells of the blood. It is also formed 

 from the breaking down of the cells of such tissues, as 

 lymphatic glands and thymus. 



Many circumstances influence the rapidity of clotting. 

 Temperature has a marked effect ; a low temperature re- 

 tarding it, a slight rise of temperature above the normal of 

 the particular animal accelerating it. If a trace of a neutral 

 salt be added to blood, coagulation is accelerated ; but if blood 

 be mixed with strong solutions of salt, coagulation is pre- 

 vented. Salts of lime have a marked and important action, 

 and if they are precipitated by the addition of oxalate of 

 soda, blood will not clot, apparently because thrombin cannot 

 be formed. 



The injection into the blood vessels of a living animal of 

 commercial peptones, which chiefly consist of proteoses, or 

 of an extract of the head of the medicinal leech, retards 

 coagulation after the blood is shed. They appear to cause 



