THE BLOOD. 173 



in coagulating power, just as a precipitin is produced as the result of 

 the injection of foreign protein. Hence it has been inferred that small 

 quantities of thrombin are continually being produced in the blood, 

 thrombokinase being set free by the breaking down of white blood 

 corpuscles and of the tissues generally. The presence of thrombin 

 leads to the formation of antithrombin in the liver, and in this way 

 clotting in the blood-vessels is prevented. 



Conditions which Accelerate Clotting. The rate of coagulation is 

 accelerated (1) by a certain degree of warmth, (2) by agitation of 

 the blood, and (3) by increasing the extent of the foreign surface 

 with which the blood is in contact. A practical application of the 

 latter fact may be made by applying a sponge or cotton wool to a 

 bleeding surface to aid in the arrest of haemorrhage. It is probable that 

 the foreign surface facilitates the formation and disintegration of 

 platelets with consequent increased production of thrombokinase. 



Intravascular Clotting. The rapid injection into the blood-stream 

 of an animal of a quantity of a saline extract of such a cellular organ 

 as the thymus or a lymph gland, leads to the coagulation of the blood 

 throughout the whole vascular system. This result is generally ascribed 

 to the presence of nucleo-protein in 'the extract. Small quantities of 

 a similar extract, slowly injected, have an opposite effect, rendering the 

 blood incoagulable. The difference in the results, according to the 

 quantity of extract injected, has not been satisfactorily explained. 

 Intravascular clotting is also produced by the injection of thrombokinase 

 or of snake venom, but not by moderate quantities of thrombin. 



Conditions which Retard or Prevent Clotting. These may be 

 classified as 



(1) Prevention of contact with a foreign surface. 



(2) Removal of one or. more of the substances concerned in the 



formation of fibrin. 



(3) Interference with -the interaction of the substances concerned 



in the formation of fibrin. 



(-4) The use of an anticoagulin, or the production of antithrombin 

 by the injection of certain substances into the blood-stream 

 before the blood is shed. 



(1) Coagulation is delayed if blood is shed into a vessel the interior 

 of which is smeared with grease of any kind. It is delayed for a 

 longer time if the blood is kept in contact with the lining of a blood- 

 vessel. If a large vein containing blood is ligatured in two places 

 and the ligatured portion is excised, the blood in the vein, which is 

 then known as a "living test-tube," may remain fluid for days. 



(2) (a) Lime salts are precipitated by the addition of potassium 



