954 PHYSIOLOGY 



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 endo- 

 thelium 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 thrombo- 

 kinase. If we disbelieve in their pre-existence the first act in coagula- 

 tion must be a change in the plasma itself (which perhaps can be 

 regarded as a dropsical protoplasm), leading to the separation 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 coagula- 

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

 the preservation of the fluidity of the circulating blood. One factor at 

 any rate in the prevention of intra vascular 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 therefore conclude 

 that mere contact with a foreign body has a direct destructive action 

 either on the plasma leading to the formation of blood-platelets, or 

 on the latter, leading to their disintegration and the discharge of 

 thrombokinase. Birds' blood, for instance, can be made to clot with- 

 out the addition of tissue juice if, by increasing the mechanical insult, 

 as by violent whipping, filtration through a clay cell, or addition of 

 water, we destroy the leucocytes and red blood-corpuscles, so leading 

 to the liberation of their contained thrombokinase. Another factor 

 is probably the presence of what we may call antithrombins in 

 circulating blood. Although evidence of the existence of these bodies 



