THE COAGULATION OF THE BLOOD 



211 



CHAPTER XIX 



THE COAGULATION OF THE BLOOD 



A. EXTRAVASCULAR CLOTTING 



Physical Changes in Coagulating Blood. Possibly the most strik- 

 ing characteristic of the mammalian blood is its power of changing its 

 fluid state into one of semisolidity. As this conversion, designated as 

 coagulation, may set in either after the blood has escaped from the 

 blood-vessels, or while still within them, two forms of clotting are 

 obtained, namely, the extravascular and the intravascular. 



FIG. 114. THE COAGULATION or THE BLOOD. 



A, Normal blood; B, the formation of fibrin from colonies of thrombocytes envelop- 

 ing the formed elements; C, the separation into the coagulum and supernatant serum. 



In normal blood the different corpuscular elements are freely sus- 

 pended in the plasma. When coagulation sets in, delicate shreds of 

 fibrin are formed which advance from certain fixed points and traverse 

 the blood in different directions, encircling large colonies of corpuscles. 

 In accordance with the view of Wooldridge, these filaments arise in 

 consequence of a deposition of fine crystals which become confluent and 

 are finally united into an extensive network. The production of fibrin, 

 therefore, is essentially a process of crystallization, so that the coagula- 

 tion of the blood may be said to be based upon the crystallization of 

 fibrin from a supersaturated solution. The physical characteristics 

 of these crystals, as well as their functional properties, make it certain 

 that they are retained in a liquid state and should therefore not be 

 considered as solids. The meshes of this network are gradually drawn 

 more closely together so that the corpuscular elements become more 

 tightly packed. The entire mass finally gravitates to the bottom of 

 the receptacle. This gelatinous deposit is known as the coagulum. 

 It is composed of fibrin, the different types of corpuscles, and nutritive 

 material. 



If the blood is permitted to clot slowly, so that a complete deposi- 

 tion of the red cells is had, the coagulum presents a marginal zone, 



