BLOOD AND LYMPH 257 



of fibrin. This fibrin forms in shreds throughout the blood mass and, 

 holding the form elements of the blood within its meshes, serves to 

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

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

 exudes. 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 reac- 

 tion, (5) preparation of hemin crystals. Of these five tests the last 

 two named are generally considered to be the most satisfactory. They 

 give equally reliable results with fresh blood and with blood from clots 

 or stains of long standing, provided the latter have not been exposed to 

 a high temperature or to the rays of the sun for a long period. The 

 technic of the tests is simple and the 'formation of the dark brown or 

 chocolate-colored crystals of hemin or the production of the green or 



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



2 Bell: Jour. Path, and BacL, 18, No. 4, 1914. 



