108 THE BLOOD AND THE LYMPH 



can be extracted from the tissues by fat-solvents, and that it appears to 

 belong to the class of phosphatids, being indeed closely related to, if 

 not identical with, kephalin (Howell). 



Intravascular Clotting 



The practical application of the theory of blood clotting concerns the 

 manner in which the blood is maintained in a fluid condition in the blood 

 vessels, and the disturbance of this function causing intravascular clot- 

 ting. According to the one theory, the blood is maintained fluid by the 

 absence from it of any considerable quantity of kinase, and according 

 to the other, by the presence in it of an amount of antithrombin suffi- 

 cient to prevent the union of calcium with prothrombin. The fluidity 

 is maintained even when large amounts of thrombin or of blood serum, 

 which contains this substance, are injected into the living animal. We 

 can best explain the immunity of the blood to the action of thrombin un- 

 der these circumstances as being due to the instantaneous appearance in it 

 of antithrombin in amounts sufficient to prevent the action of thrombin 

 on fibrinogen, for, as stated above, it is claimed by Howell that anti- 

 thrombin has this influence as well as that of preventing the conversion 

 of prothrombin into thrombin. 



Intravascular clotting may be brought about by a variety of means: 

 (1) Mechanical damage to the lining of the blood vessels; after the ap- 

 plication of a ligature, for example, the damaged endothelium is soon 

 covered by a clot, which gradually becomes firmer and firmer, and may 

 spread up the vessel to the next branch. (2) The presence of foreign 

 substances in the bloQd. Emboli, for example, are apt to cause clots 

 to form at the places where they stick, namely, in the smaller vessels. 

 Clotting is also a frequent occurrence when there are local dilatations of 

 the cardiovascular tube, and it may occur under imperfectly understood 

 conditions causing the condition known as thrombosis. (3) An inter- 

 esting variety of intravascular clotting is that caused by the intrave- 

 nous injection of saline extracts of cell-rich tissues, such as the thymus, 

 lymph glands or testes (Wooldridge). By precipitation with acetic 

 acid and digestion with peptone, a residue can be obtained from these 

 extracts which, when dissolved in alkali, has a very pronounced intra- 

 vascular clotting effect. Since these precipitates are very rich in phos- 

 phorus, it is probable that they are of the nature of phosphoprotein 

 (nucleoalbumin). Their action must depend on neutralization of anti- 

 thrombin, according to HowelPs theory, or because they serve as throm- 

 bokinases (according to Morawitz' theory). 



As a matter of fact, however, the foregoing observation is not com- 

 pletely explained by either theory. If in place of making one injection 



