888 PHYSIOLOGY 



blood leaves the vessels, the presence of such corpuscles in the higher animals 

 has not been demonstrated, though in some invertebrata they are certainly 

 present. The sole source therefore of the thrombokinase is the very perish- 

 able formed elements of which we have spoken as the blood platelets. The 

 very existence of this element is doubtful in normal blood. What is certain 

 is that any slight change in the plasma, whether due to contact with a foreign 

 object or to cooling of the blood, causes the appearance of these elements. 

 The first act therefore in coagulation is the appearance of the blood platelet 

 and its disintegration with the setting free of thrombokinase. The part 

 played by the platelets in coagulation is of great importance in maintaining 

 the integrity of the vascular system. If a fine needle be thrust through the 

 wall of a venous capillary which is kept under observation by the microscope, 

 it will be seen that the blood, as it flows past the injured spot, deposits blood 

 platelets on the side of the puncture. These aggregate to form a plug closely 

 adherent to the wall of the vessel, which effectively prevents any escape of 

 the contents of the capillary. The blood platelets fuse so as to form a mass 

 of fibrin, and later on by the growth of the adjacent endothelial cells the 

 thrombus is organised, converted into connective tissue, and covered with 

 a layer of endothelium continuous with the rest of the vessel. The same 

 process occurs when any part of the lining membrane of a large vessel is 

 injured. Thus destruction of a patch of endothelium in a vein leads to 

 the deposition of blood platelets over the patch and the formation of a 

 ' thrombus ' adherent to the wall. From this thrombus coagulation may 

 spread through the rest of the contents of the vessel and produce thrombosis 

 of the whole vein. Under healthy conditions the thrombus serves simply 

 to cover the bare area in the wall of the vein and is grown over later by 

 endothelium, so restoring the integrity of the vessel wall. If we believe 

 in the pre-existence of blood platelets in the circulating blood, we must 

 assume the first act in coagulation to be the disintegration of these elements 

 and the setting free of thrombokinase. If we disbelieve in their pre-existence, 

 the first act in coagulation must be a change in the plasma itself (which 

 perhaps can be regarded as a dropsical protoplasm), leading to the separa- 

 tion of an unstable substance, thrombokinase, in the form of a disc-like 

 precipitate which rapidly undergoes further changes, reacting with the 

 thrombogen remaining in solution in the plasma with" the production of fibrin 

 ferment. 



Why does the blood not clot in the vessel ? No theory of coagulation 

 can be satisfactory which does not account at the same time for the preserva- 

 tion of the fluidity of the circulating blood. One factor at any rate in the 

 prevention of intravascular clotting must be the nature of the surfaces with 

 which the blood comes in contact. The blood, even of mammals, can be 

 prevented for a time from clotting if it be kept carefully from contact with 

 any foreign substance which is wetted by it, as for instance when it is 

 received into vessels free from dust and coated with a layer of oil or paraffin. 

 On the other hand, free contact with such substances, as occurs when the 

 blood is whipped, materially hastens the process of coagulation. One must 



