420 BLOOD AND LYMPH. 



may assume that in the living blood-vessels the formed elements 

 leucocytes and blood plates do not disintegrate in great numbers 

 at a time, and therefore do not give rise to any noticeable amount of 

 active thrombin. It seems most probable that little, if any, throm- 

 bin is actually present in the blood under normal circumstances, 

 and this in itself may be regarded as the main reason for the fact 

 that the blood remains unclotted. It is quite possible, however, 

 that other safeguards may exist in a matter of such prime im- 

 portance. It has been shown, for instance, that, when solutions 

 of fibrin ferment (thrombin) are injected into the circulation, 

 clotting is not produced with the certainty that one might expect. 

 Delezenne has described experiments which indicate that the liver 

 exercises a defensive power in this respect.* He states that when 

 blood-serum containing, as it normally does, an active thrombin 

 - is circulated through a living liver it loses its power of inducing 

 coagulation in solutions containing fibrinogen. Its thrombin has 

 been destroyed or made inactive by some effect of the liver, and it is 

 possible, although not demonstrated as yet, that the liver may 

 exercise such a protective action under special circumstances during 

 life. That this supposed action of the liver is not always essential 

 is shown by the fact that in animals from whom the liver has been 

 removed experimentally the blood does not clot within the vessels. 

 The older observers were impressed with the fact that blood remains 

 uncoagulated for long periods if kept in contact with what may be 

 called its normal surface, that is, the interior of the heart or blood- 

 vessels. In an excised heart or blood-vessel the blood, although at 

 rest, remains fluid for a long time. It was thought possible, there- 

 fore, that the normal endothelial walls of the vessels exercise a 

 restraining influence of some kind upon the coagulation of the blood. 

 In recent times this view has taken the form, corresponding to the 

 knowledge of the day, of a suggestion that an antibody namely, 

 an antithrombin exists in the blood and actively retards or prevents 

 coagulation. While some authors (Morawitz) believe that such an 

 antithrombin exists normally in circulating blood and is essential 

 in maintaining its fluidity, others (Schmidt) hold to the view that 

 substances retarding coagulation are liberated only from the dis- 

 integration of the cellular elements and are present practically, 

 therefore, only in the shed blood. Loebf has not been able to 

 detect the existence of an antithrombin in extracts of the inner wall 

 of the blood-vessels. It would seem to be premature to accept 

 the view that under normal conditions there exists in the blood any 

 substance that retards or prevents coagulation, although under 



* " Travaux de Physiologie," Universite de Montpellier, 1898. 

 f Leo Loeb, "Virchow's Archiv," 176, 10, 1904; also " Hof meister's Bei- 

 trage," 5, 534, 1904. 



