COAGULATION. 463 



that point — there may be an agghitination of the blood plates, 

 starting at the injured surface, and the subsequent formation of a 

 clot. Intravascular clotting may also be produced by the injection 

 of various substances. From our knowledge of the factors con- 

 cerned in coagulation we should suppose that the normal balance 

 of the blood might be overthrown in one of two general ways, 

 by injecting active thrombin or by injecting extracts of the tissues 

 (thromboplastic substance). As a matter of fact, intravascular 

 clotting may be produced by either of these methods, but experi- 

 ments have developed the somewhat unexpected result that the 

 blood or the body can protect itself within wide limits from the 

 effects of such injections. Solutions of pure thrombin, or serum 

 or defibrinated blood containing active thrombin, may be intro- 

 duced into the circulation in quantities that outside would suffice 

 to clot the whole mass, and yet no harm be done. The explana- 

 tion of this negative result, according to the author's experiments, 

 is that the excess of thrombin within the living animal causes the 

 formation of a compensatory amount of antithrombin, probably 

 by a Drotective reflex secretion. Tissue extracts or solutions of 

 the precipitated thromboplastic substance (Wooldridge) cause 

 clotting more readily, but here again injections of this kind, instead 

 of hastening the clotting of blood, may at times have just the 

 opposite effect, giving what older observers called the ''negative 

 phase of the injection." On the other hand, when the coagulability 

 of the blood is diminished experimentally by injecting antithrom- 

 bin (hirudin) or peptone solutions the effect soon passes off. It 

 may be pointed out that these attempts to devise methods by 

 means of which the coagulability of the blood may be controlled 

 have a practical Ijearing in their application to pathological con- 

 ditions in which the balance is already disturbed in the direction 

 of an increased or decreased coagulability. 



Means of Hastening or of Retarding Coagulation. — Blood 

 coagulates normally within a few minutes, but the process may be 

 hastened by increasing the extent of foreign surface with which it 

 comes in contact. Thus, agitating the liquid when in quantity, or 

 the application of a sponge or a handkerchief to a wound, hastens 

 the onset of clotting. This is easily understood when it is remem- 

 bered that the breaking clown of leucocytes and blood-plates is 

 hastened by contact with foreign surfaces. It has been proposed 

 also to hasten clotting in case of hemorrhage by the use of throm- 

 bin solutions or of tissue extracts containing some thromboplastic 

 substance. Hot sponges or cloths applied to a wound hasten 

 clotting, probably by accelerating the formation of thrombin and 

 the chemical changes of clotting. Coagulation may be retarded 



