38 THE CIRCULATING LIQUIDS OF THE BODY 



peptone plasma must be assumed to contain both thrombogen and 

 thrombokinase, since it can be made to clot in various ways (e.g., by dilu- 

 tion with water or by slight acidulation with acetic acid) without the 

 addition of anything which could supply either of these factors. Yet 

 a little tissue extract causes it to clot much more rapidly than simple 

 dilution or acidulation, and more rapidly than the addition of serum. 

 So that either the thrombokinase already present in peptone plasma is 

 present in an unavailable form, or in some way the formation of throm- 

 bin from its precursors is hindered. But this is not the only cause of 

 the incoagulability of peptone plasma. It may be shown to contain 

 an antithrombin, a body which antagonizes the action of fully formed 

 thrombin, and which does not seem to be a ferment, since it acts quan- 

 titatively in proportion to the amount present. This is the reason why, 

 although peptone plasma can always be made to clot by the addition 

 of fibrin ferment, in serum, for instance, relatively large quantities of 

 it must be supplied (Practical Exercises, pp. 64, 65). 



Fig. 5. Fibrin Formation in Horse's Plasma (Ultramicroscope) (Stiibel). Several 

 clumps of disintegrated platelets from which the fibrin filaments radiate. 



An extract of the head of the medicinal leech in salt solution prevents 

 the clotting of blood both in the test-tube and when injected into the 

 circulation. The plasma obtained differs from peptone plasma in 

 refusing to coagulate unless tissue extract is added. It is therefore 

 deficient in thrombokinase, or, rather, as has been shown, the kinase 

 present is unable to act, because neutralized by antikinase present in the 

 leech extract. Leech extract also contains an antithrombin, which can 

 be neutralized by a sufficient amount of thrombin. In the small 

 wound from which the leech sucks blood this sufficient amount is not 

 present, and the blood remains unclotted, as it also does in the alimen- 

 tary canal of the leech. The anticoagulant substance, hirudin, has 

 been isolated, and gives the reactions of an albumose. 



Sources of Thrombogen and Thrombokinase. It has already been 

 stated that thrombogen exists in the circulating plasma. This is 

 shown by the fact that fluoride plasma obtained from blood drawn 

 directly through a wide cannula into sodium fluoride solution, with 



