260 PHYSIOLOGICAL CHEMISTRY 



This fibrin forms in shreds throughout the blood mass and, hold- 

 ing the form elements of the blood within its meshes, serves to pro- 

 duce the typical blood clot. The fibrin shreds gradually contract, the 

 clot assumes a jelly-like appearance and the yellowish serum ex- 

 udes. If, immediately upon the withdrawal of blood from the body, 

 the fluid be rapidly stirred or thoroughly " whipped" with a bundle of 

 coarse strings, twigs or a specially constructed beater, the fibrin shreds 

 will not form in a network throughout the blood mass but instead will 

 cling to the device used in beating. In this way the fibrin may be 

 removed and the remaining fluid is termed defibrinated blood. The 

 above theory of the coagulation of the blood may be stated briefly as 

 follows: 



I. Prothrombin + Calcium Salts = Thrombin (or Fibrin-ferment) . 



II. Thrombin (or Fibrin-ferment) + Fibrinogen = Fibrin. 

 Howell 1 has suggested an ingenious modification of the above theory. 



He says: "In the circulating blood we find as constant constituents 

 fibrinogen, prothrombin, calcium salts and antithrombin. The last-named 

 substance holds the prothrombin in combination and thus prevents its 

 conversion or activation to thrombin. When the blood is shed, the 

 disintegration of the corpuscles (platelets) furnishes material (throm- 

 boplastin) which combines with the antithrombin and liberates the 

 prothrombin; the latter is then activated by the calcium and acts on 

 the fibrinogen. According to this view the actual process of coagula- 

 tion involves only three factors, fibrinogen, prothrombin and calcium. 

 These three factors exist normally in the circulating blood but are 

 prevented from reacting by the presence of antithrombin." 



The question as to whether menstrual blood coagulates has caused 

 much discussion. The most recent investigations seem to show that it 

 does not coagulate because of the removal of fibrin-ferment and fibrinogen 

 from such blood by the endometrium or lining membrane of the uterus. 2 



Among the medico-legal tests for blood are the following: (i) 

 Microscopical identification of the erythrocytes, (2) spectroscopic iden- 

 tification of blood solutions, (3) the guaiac test, (4) the benzidine 

 reaction, (5) preparation of hemin crystals, (6) Bordet reaction 

 ("biological" blood test). This last test is the most satisfactory 

 medico-legal test for blood. 



Up to within recent times it has been impossible to make an absolute 

 differentiation of human blood. The so-called "biological" blood test 

 has, however, made such a differentiation possible. This test, known as 

 the Bordet reaction, is founded upon the fact that the blood serum of an 



1 Howell: American Journal of Physiology ; 29, 187, 1911. " 



*Bell: Jour. Path, and Bad., 18, No. 4, 1914. 



