318 THE HUMAN BODY 



glass tube with care to avoid contamination from the wound clot- 

 ting takes place very slowly, or in birds and reptiles may not occur 

 at all. The deduction is that the tissues over which the blood flows 

 in ordinary hemorrhage contain something that neutralizes the 

 antithrombin. For this the name thromboplastic substance has been 

 suggested. The leucocytes, and probably also the platelets, of 

 mammalian blood contain enough of this substance so that their 

 disintegration will neutralize the antithrombin and allow clot- 

 ting to occur even though the blood may not have come at all 

 into contact with the tissues. In ordinary bleeding, however, 

 the escaping blood must flow directly over the raw tissue sur- 

 faces so that the antithrombin is promptly neutralized and 

 clotting can proceed at once. Apparently all tissues contain 

 thromboplastic substance except those that form the lining 

 membranes of the blood-vessels. This rather cumbersome mech- 

 anism appears to be necessary to insure prompt clotting when 

 the blood-vessels are ruptured and at the same time immunity 

 from the disaster of clot-formation within the circulation. 



The formation of blood-clots (thrombi) within the vessels is 

 likely to be followed by serious effects, due to the plugging of 

 important vessels by the clotted blood, but the occurrence of 

 thrombi in the intact healthy circulation is unknown; their forma- 

 tion presupposes some injury to the walls of the blood-vessels, as 

 by crushing them or tying ligatures about them. 



Summary of the Process of Coagulation. We may picture the 

 entire process of blood-clotting somewhat as follows: 



1. As the result of rupture of the blood-vessels and contact of 

 the blood with raw tissue surfaces the antithrombin is neutralized 

 by thromboplastic substance and the blood-plates disintegrate, 

 yielding prothrombin. 



2. The prothrombin thus set free reacts with the calcium of the 

 blood and forms thrombin. 



3. By a reaction between thrombin and fibrinogen insoluble 

 fibrin is precipitated in the form of a sticky network. 



4. The fibrin network entangles corpuscles within it, forming a 

 typical clot. 



Methods of Hastening or Retarding Coagulation. Since the 

 process of clotting is in several steps there are a corresponding 

 number of points at which its normal course may be broken into, 



