Chap. IX] VASCULAR SYSTEM 151 



(4) Injury to the walls of the blood-vessels. 



(5) Rest. 



It is generally conceded that the first four conditions hasten the 

 disintegration of blood-plates and white corpuscles, and conse- 

 quently the formation of thrombin and of a clot. Many standard 

 text-books include agitation instead of rest. If blood is contained 

 in a vessel, agitation of it does hasten the process of clotting in 

 the same way that the first four conditions do. If, however, we 

 consider the formation of a clot in the open end of an injured 

 vessel, we keep the part at rest, because agitation might dislodge 

 the clot after it had formed. 



Clotting is hindered by : — 



(1) A very low temperature. Cold hinders the formation of a 

 clot but is often used to check hemorrhage, because it stimulates 

 the blood-vessels to contract. 



(2) Contact with living tissues, especially the walls of the blood- 

 vessels. 



(3) The addition of strong acids or alkalies, neutral salts, oil 

 or other viscid substances, certain organic ferments, or a large 

 quantity of water. 



(4) Absence of calcium salts. 



(5) Absence of fibrinogen. 



(6) Removal of fibrin. If fresh blood, before it has time to 

 clot, be whipped with a bundle of twigs, the fibrin will form on 

 the twigs, and if the whipping of the blood be continued until 

 after the fibrin has been deposited on the twigs, the blood left in 

 the vessel will be found to have lost the power of clotting. Such 

 blood is called defibrinated. 



Why blood does not clot within the blood-vessels. — Fortunately 

 blood rarely clots within the blood-vessels during life. Why it 

 does not is not known, but two theories are advanced to account 

 for it. 



(1) Circulating blood does not contain thrombin because the 

 blood-plates and white corpuscles do not disintegrate in sufficient 

 numbers to allow for the formation of it. 



(2) Circulating blood does contain some thrombin, but it 

 also contains a substance called antithrombin which is secreted 

 by the lining of the heart and blood-vessels. Antithrombin 

 neutralizes or prevents the activity of thrombin. 



