CHAP, i.] BLOOD. 29 



That the blood within the living blood vessels, though not 

 actually clotting under normal circumstances, may easily be made 

 to clot, that the blood is in fact so to speak always on the point 

 of clotting, is shewn by the fact that a foreign body, such as a 

 needle thrust into the interior of a blood vessel or a thread drawn 

 through and left in a blood vessel, is apt to become covered with 

 fibrin. Some influence exerted by the needle or thread, whatever 

 may be the character of that influence, is sufficient to determine a 

 clotting, which otherwise would not have taken place. 



The same instability of the blood as regards clotting is strikingly 

 shewn, in the case of the rabbit at least, by the result of injecting 

 into the blood vessels a small quantity of a solution of a peculiar 

 proteid prepared from certain structures such as the thymus body. 

 Massive clotting of the blood in almost all the blood vessels, small 

 and large, takes place with great rapidity, leading to the sudden 

 death of the animal. In contrast to this effect may be mentioned 

 the result of injecting into the blood vessels of a dog a quantity 

 of a solution of a body called albumose, of which we shall hereafter 

 have to treat as a product of the digestion of proteid substances, 

 to the extent of '3 grm. per kilo of body weight. So far from 

 producing clotting, the injected albumose has such an effect on 

 the blood that for several hours after the injection shed blood will 

 refuse to clot of itself and remain quite fluid, though it can be 

 made to clot by special treatment. 



23. All the foregoing facts tend to shew that the blood 

 as it is flowing through the healthy blood vessels is, so far as 

 clotting is concerned, in a state of unstable equilibrium, which 

 may at any moment be upset, even within the blood vessels, 

 and which is upset directly the blood is shed, with clotting 

 as a result. Our present knowledge does not permit us to 

 make an authoritative statement as to the exact nature of this 

 equilibrium. There are reasons however for thinking that the 

 white corpuscles play an important part in the matter. Where- 

 ever clotting occurs naturally, white corpuscles are present ; and 

 this is true not only of blood but also of such specimens of peri- 

 cardial or other serous fluids as clot naturally. And many argu- 

 ments, which we cannot enter upon here, may be adduced all 

 pointing to the same conclusion, that the white corpuscles play 

 an important part in the process of clotting. But it would lead 

 us too far into controversial matters to attempt to define what 

 that part is, or to explain the exact nature of the equilibrium of 

 which we have spoken. What we do know is that in blood soon 

 after it has been shed, the body which we have called fibrinogen 

 is present as also the body which we have called fibrin ferment, 

 that the latter acting on the former will produce fibrin, and that 

 the appearance of fibrin is undoubtedly the cause of what is called 

 clotting. We seem justified in concluding that the clotting of 

 shed blood is due to the conversion by ferment of fibrinogen into 



