284 THE HUMAN BODY 



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. 



It should be stated in this connection that some physiologists 

 have supposed the process of blood-clotting to be more complex 

 than here outlined. Their view has been based upon such observa- 

 tions as that blood drawn carefully from blood-vessels without 

 coming into contact with the tissues of the wound clots much 

 more slowly than does blood which has flowed over raw tissue 

 surfaces. Reptiles and birds show this difference very strikingly. 

 This phenomenon has been interpreted as indicating that a third 

 substance, furnished by tissues generally, is involved in the coag- 

 ulation process. The tendency of the most recent work seems, 

 however, to be against the necessity of there being any such 

 third substance. In the case of birds, for instance, it has been 

 shown that clotting is brought on as quickly by throwing a little 

 dust into the blood as by allowing it to come in contact with raw 

 tissues; a result that removes the probability of the tissue yield- 

 ing a specific substance to the coagulation process. 



Why Coagulation Does Not Occur within the Blood- Vessels. 

 In the process of clotting outlined above the initial step is the 

 disintegration of the blood-plates; unless this takes place there 

 will be no clotting. It is only in the presence of a foreign surface, 

 such as air or the injured wall of a blood-vessel, that there is any 

 considerable breaking down of these structures. There is, doubt- 

 less, a certain amount of disintegration occurring normally in the 

 blood-stream, for there is no reason to suppose that the blood- 

 plates are specially endowed with longevity, but the small amounts 

 of prothrombin thus liberated and the thrombin which may be 

 formed therefrom are probably taken care of either by the cells 

 forming the lining of the blood-vessels or by some other struc- 

 tures adapted to that purpose; there is some evidence that the 

 liver functions in this way. 



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

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

 portant vessels by the clotted blood, but the occurrence of thrombi 

 in the intact healthy circulation is unknown; their formation pre- 

 supposes either some injury to the walls of the blood-vessels, as 



