v THE BLOOD: PLASMA 137 



coagulation, unlike that required in milk coagulation, is minimal, 

 but under uniform experimental conditions it is constant ; we thus 

 have a critical value for the concentration of Ca-ions, below which 

 the blood remains indefinitely liquid. 



On the one hand, accordingly, all physical or chemical agents 

 that lower the concentration of the Ca-ions of the blood below the 

 critical value provoke incoagulability ; on the other, all those 

 agents which raise it above the said value favour coagulation. 

 Among the former are cold, high molecular concentration, small 

 doses of reagents which form almost insoluble salts with calcium 

 (oxalates, fluorides, soaps, carbonates, alkaline pyrophosphates), 

 moderate doses of reagents which with lime form simple salts that 

 are sparingly soluble (di-sodic sulphate and phosphate, sodium 

 bicarbonate), small doses of reagents winch with lime form 

 compounds that are little ionisable (tri-sodic citrate, sodic meta- 

 phosphate) ; among the latter are heat, dilution with water, addition 

 of small quantities of ionisable lime salts, addition of reagents that 

 liberate the calcium from its insoluble or little dissociable compounds. 



On the other hand, the addition of small quantities of calcium 

 to normal blood invariably diminishes coagulability (Sabbatani, 

 Eegoli), and it was only the inexact interpretation of certain 

 experiments of Dastre and Hammarsten that led people for some 

 time to believe that it was increased by the same ; the addition 

 of a moderate quantity much delays coagulation : large doses 

 entirely prevent it (Home). 



From all these results it appears, in regard to the concentration 

 of Ca-ions in the blood, that we must assume an optimum value 

 for coagulation (lying between the limits of the physiological 

 Variations which calcium presents in normal blood), and two 

 critical values, minimal and maximal, above and below which the 

 blood no longer coagulates. 



The addition of lime increases coagulation only when the blood 

 is deficient in it. 



(/) Next conies the question of determining by what process 

 the injury or dissociation of blood corpuscles leads to the breaking- 

 up of fibrinogen into fibrin and fibrin-globulin. 



A. Schmidt and his school (Dorpat) treat blood coagulation as a 

 process determined by an enzyme which they call thrombin, which is 

 derived from the blood corpuscles, particularly from the leucocytes 

 and platelets. In the normal state these contain, not the ferment, 

 but a zymogenic substance, pro-thrombin, which on injury or 

 destruction of the corpuscles gives rise to the ferment proper, 

 tkrombin. This can be extracted either from defibrinated blood or 

 from blood serum, by absolute alcohol, which precipitates the 

 protein matters with the ferment. The ferment can be extracted 

 from the mass of the well-dried and pulverised clot by making a 

 watery extract. 



