BLOOD 479 



when shed. It appears to be of the nature of an anti- 

 thrombin. 



The blood does not coagulate in the vessels under normal 

 conditions because of the absence of thromboplastin in any 

 quantity and the presence of anti-thrombin. Under certain 

 conditions clotting does take place. (1) If inflammation is 

 induced in the course of a vessel, coagulation occurs rapidly. 

 (2) If the inner coat of a vessel be torn, as by a ligature, or 

 if any roughness occurs on the inner Avail of a vessel, 

 coagulation is apt to be set up. (3) Various substances 

 injected into the blood stream may cause the blood to 

 coagulate, and thus rapidly kill the animal. Among such 

 substances are extracts of various organs — thymus, testis, 

 and lymph glands, which yield thromboplastin — and snake 

 venom, which seems to contain active thrombin. The 

 injection of pure thrombin does not usually cause clotting, 

 apparently because an anti-thrombin is developed to neutral- 

 ise it. 



The blood usually clots when shed, because the damaged 

 tissues yield thromboplastin, and thus thrombin is formed. 

 This acts upon the fibrinogen before it can be antagonised 

 by the anti-thrombin. If blood is received into oil, 

 or into a vessel anointed with vaseline and filled with 

 paraflSn oil, it will remain fluid for a considerable time. 

 Any roughness in the wall of the blood-vessel or of the 

 vessel in which the blood is received probably serves to 

 catch the blood platelets (p. 484), so that thromboplastin is 

 liberated freely as they disintegrate. 



III. Plasma and Serum. 



These may be considered together, since serum is merely 

 plasma minus fibrinogen. As serum is so much easier to 

 procure, it is generally employed for examination, but 

 plasma may be readily obtained by centrifuging blood which 

 has been prevented from clotting by the addition of an 

 oxalate or a citrate (p. 477). 



Both are straw-coloured fluids, the colour being due to a 

 yellow lipochrome. Sometimes they are clear and trans- 



