THE BLOOD 



leucocytes and blood platelets after shedding. In the presence 

 of calcium, thrombokinase converts thrombogen into thrombin. 



Not only is thrombokinase present in the above cellular 

 elements of the blood, but it exists in all the tissues of the body. 

 The practical importance in the case of a wound of having 

 thrombokinase at hand to act on the thrombogen, and so cause 

 coagulation by the production of thrombin, is very evident, for 

 the cessation of haemorrhage is due to the formation of fibrin 

 in the mouths of the vessels. 



There is a substance known as ' nucleo-protein,' readily 

 obtained from tissue-cells, thymus, kidney, lymphatic glands, 

 and other organs, which is very closely identified with thrombin. 

 If nucleo-protein be prepared, and a large dose 

 injected into the veins of an animal, intra- 

 vascular clotting and death at once occur. On 

 the other hand, if a small dose be injected, no 

 such effect is produced — in fact, the blood is 

 rendered uncoagulable. To this phenomenon the 

 term ' negative phase ' has been applied, in con- 

 tradistinction to the positive phase in which 

 clotting at once occurs. The explanation which 

 has been offered of the negative phase is that an 

 antibody is produced which neutralizes the throm- 

 bokinase. But it has also been held that two 

 different substances are obtainable from the above 

 tissue-cells — one, htconitclein, which accelerates, 

 and the other, histon, which retards coagulation ; 

 whichever effect is observed is said to depend 

 upon the relative amount of each present. 



The view that an antibody exists in living 

 blood which prevents coagulation in the vessels 

 is urged by some. Such an antithrombin has 

 not at present been extracted from the inner 

 wall of the bloodvessels, but the theory offers an explanation, not 

 only of why blood remains fluid during life, but also of the fact 

 that coagulation of the blood in the vessels after death is a slow 

 process. It also offers a reasonable explanation of a very old 

 experiment on the horse, in which the jugular veins, occluded by 

 ligatures and excised, maintains the enclosed blood in a fluid 

 condition for one or two days, so long as it is left in contact with 

 the wall of the vessel ; clotting, nevertheless, at once occurs on 

 removal. Fig. 8 shows diagrammatically this so-called * living 

 test-tube ' experiment, the explanation of which was for years 

 attributed to the influence exerted in some way or other by the 

 normal endothelium of the vessel on the contained blood. 



Intravascular clotting may occur under pathological conditions, 



Fig. 



OF 



TIED 

 TWEEN 



8.— Vein 



a Horse 



b E- 



Two 



Ligatures. 



Plasma ; 2, 

 white corpus- 

 cles ; 3, red 

 corpuscles. 



