becomes converted into fibrin. The experimental study of the rate of fibrin formation, when different strengths of thrombin solutions are allowed to act upon a fibrinogen solution, leads us to the probable conclusion that the first of these three possibilities is the correct one, and that thrombin therefore exerts a true ferment action upon fibrinogen. It is known that in the reaction, in addition to the formation of fibrin, yet another protein makes its appearance. This is known as fibrinoglobulin, and apparently it arises from the fibrinogen, so that the change would be one of cleavage into fibrin and fibrinoglobulin. It is very noteworthy that although the amount of fibrin formed during the clotting appears very bulky, yet the actual weight is extremely small, not more than 0.4 grms. from 100 cc. of blood.
Having ascertained that the clotting is due to the action of thrombin upon fibrinogen, we now see that the next step to be explained is the origin of thrombin. It has been shown that the final step in its formation consists in the combination of another substance, termed prothrombin, with calcium. Any soluble calcium salt is found to be effective in this respect, and conversely the removal of soluble calcium (e.g. by sodium oxalate) will prevent the formation of thrombin and therefore of clotting.
In the next place it can be proved that prothrombin does not exist as such in circulating blood, so that the problem becomes an inquiry as to the origin of prothrombin. Experiment has shown that in its turn prothrombin arises from yet another precursor, which is named thrombogen, and that thrombogen also is not to be found in circulating blood but only makes its appearance after the blood is shed. The conversion of thrombogen into prothrombin has been proved to be due to the action of a second ferment which has been named thrombokinase, and this latter is again absent from living blood. Hence the question arises, whence are derived thrombogen and thrombokinase? In the study of this question it has been found that if the blood of birds be collected direct from an artery through a perfectly clean cannula into a clean and dust-free glass vessel, it does not clot spontaneously. The plasma collected from such blood is found to contain thrombogen but no thrombokinase. A somewhat similar plasma may be prepared from a mammal’s blood by collecting samples of blood from an artery into vessels which have been thoroughly coated with paraffin, though in this instance thrombogen may be absent as well as thrombokinase. If plasma containing thrombogen but no thrombokinase be treated with a saline extract of any tissues it will soon clot. The saline extract contains thrombokinase. This ferment can therefore be derived from most tissues, including also the white blood corpuscles and the platelets. Thrombogen is produced from the leucocytes, but it is not yet certain whether it is also formed from the platelets. The discovery of the origin of the thrombokinase from tissue cells explains a fact that has long been known, namely, that if in collecting blood, it is allowed to flow over cut tissues, clotting is most markedly accelerated. The fact that birds’ blood if very carefully collected will not clot spontaneously tends to prove that thrombokinase is not derived from the leucocytes, and makes probable its origin from the platelets, for it is known that birds’ blood apparently does not contain platelets, at any rate in the form in which they are found in mammalian blood. When examining the general properties of platelets, attention was drawn to the remarkably rapid manner in which they undergo change on coming into contact with a foreign surface. It is apparently the actual contact which initiates these changes, changes which are fundamentally chemical in character, resulting in the production of thrombokinase and possibly also of thrombogen.
Thus as our knowledge at present stands the following statement gives a recapitulated account of the changes which constitute the many phases of clotting. When blood escapes from a blood-vessel it comes into contact with a foreign surface, either a tissue or the damaged walls of the cut vessel. Very speedily this contact results in the discharge of thrombogen and thrombokinase, the former from the white blood corpuscles and also possibly from the platelets, the latter from the platelets or from the tissue with which the blood comes in contact. The interaction of these two bodies next results in the formation of prothrombin, which, combining with the calcium of any soluble lime salt present, forms thrombin or fibrin-ferment. The last step in the change is the action of thrombin upon fibrinogen to form fibrin, and the clot is complete.
The intrinsic value to the animal of these changes is quite plain. The power of clotting and thus stopping haemorrhage is of essential importance, and yet this clotting must not occur within the living blood-vessels, or it would speedily result in death. That the tissues should be able to accelerate the process is of very obvious value. That the inner lining of the blood-vessels does not act as a foreign tissue is possibly due to the extreme smoothness of their surface.
Further, an animal must always be exposed to a possible danger in the absorption of some thrombin from a mass of clotted blood still retained within the body, and we know that if a quantity of active ferment be injected into the blood-stream intravascular clotting does result. Under all usual conditions this is obviated, the protective mechanism being of a twofold character. First, it is found that thrombin becomes converted very quickly into an inactive modification. Serum, for instance, very quickly loses its power of inducing clotting in fibrinogen solutions. Secondly, the body has been found to possess the power of making a substance, antithrombin, which can combine with thrombin forming a substance which is quite inactive as far as clotting is concerned. Finally, there is evidence that normal blood contains a small quantity of this substance, antithrombin, and that under certain conditions the amount present may be enormously increased. (T. G. Br.)
Pathology of the Blood.
The changes in the blood in disease are probably as numerous and varied as the diseases which attack the body, for the blood is not only the medium of respiration, but also of nutrition, of defence against organisms and of many other functions, none of which can be affected without corresponding alterations occurring in the circulating fluid. The immense majority of these changes are, however, so subtle that they escape detection by our present methods. But in certain directions, notably in regard to the relations with micro-organisms, changes in the blood-plasma can be made out, though they are not associated in all cases with changes in the formed elements which float in it, nor with any obvious microscopical or chemical alterations.
The phenomena of immunity to the attacks of bacteria or their toxins, of agglutinative action, of opsonic action, of the precipitin tests, and of haemolysis, are all largely dependent on the inherent or acquired characters of the blood serum. It is a commonplace thatImmunity. different people vary in their susceptibility to the attacks of different organisms, and different species of animals also vary greatly. This “natural immunity” is due partly to the power possessed by the leucocytes or white blood corpuscles of taking into their bodies and digesting or holding in an inert state organisms which reach the blood—phagocytosis,—partly to certain bodies in the blood serum which have a bactericidal action, or whose presence enables the phagocytes to deal more easily with the organisms. This natural immunity can be heightened when it exists, or an artificial immunity can be produced in various ways. Doses of organisms or their toxins can be injected on one or several occasions, and provided that the lethal dose be not reached, in most cases an increased power of resistance is produced. The organisms may be injected alive in a virulent condition, or with their virulence lessened by heat or cold, by antiseptics, by cultivation in the presence of oxygen, or by passage through other animals, or they may first be killed, or their toxins alone injected. The method chosen in each case depends on the organism dealt with. The result of this treatment is that in the animal treated protective substances appear in the serum, and these substances can be transferred to the serum of another animal or of man; in other words the active immunity of the experimental animal can be translated into
