68 HANDBOOK OF PHYSIOLOGY. 



the roughness has been produced by inflammation, as in endocarditis, 

 and it is also equally true that aneurysms of arteries are sometimes spon- 

 taneously cured by the deposition within them, layer by layer, of fibrin 

 from the blood stream, which natural cure it is the aim of the physician 

 or surgeon to imitate. 



(2) Blood will remain liquid under certain conditions outside the 

 body, without the addition of any reagent, even if exposed to the air at 

 the ordinary temperature. It is well known that blood remains fluid in 

 the body for some time after death, and it is only after rigor mortis has 

 occurred that the blood is found clotted. It has been demonstrated by 

 Hewson, and also by Lister, that if a large vein in the horse or similar 

 animal be ligatured in two places some inches apart, and after sometime 

 be opened, the blood contained within it will be found fluid, and that 

 coagulation will occur only after a considerable time. But this is not 

 due to occlusion from the air simply. Lister further showed that if the 

 vein with the blood contained within it be removed from the body, and 

 then be carefully opened, the blood might be poured from the vein into 

 another similarly prepared, as from one test-tube into another, thereby 

 suffering free exposure to the air, without coagulation occurring as long 

 as the vessels retain their vitality. If the endothelial lining of the vein, 

 however, be injured, the blood will not remain liquid. Again, blood 

 will remain liquid for days in the heart of a turtle, which continues to 

 beat for a very long time after removal from the body. 



Any theory which aims at explaining the normal fluidity of the blood 

 within the living body must reconcile the above apparently contradictory 

 facts, and must at the same time be made to include all other known 

 facts concerning coagulation. We may therefore dismiss as insufficient 

 the following: that coagulation is due to exposure to the air or oxygen; 

 that it is due to the cessation of the circulatory movement; that it is due 

 to evolution of various gases, or to the loss of heat. 



Two theories, those of Lister and Briicke, remain. The former sup- 

 poses that the blood has no natural tendency to clot, but that its coagu- 

 lation out of the body is due to the action oi foreign matter with which 

 it happens to be brought into contact, and in the body to conditions of 

 the tissues which cause them to act towards it like foreign matter. The 

 latter, on the other hand, supposes that there is a natural tendency on 

 the part of the blood to clot, but that this is restrained in the living 

 body by some inhibitory power resident in the walls of the containing 

 vessels. 



The blood must contain all the substances from which fibrin is formed, 

 and the re-arrangement of these substances must occur very quickly 

 whenever the blood is shed, and so it is somewhat difficult to prevent 

 coagulation. It seems more reasonable to hold, therefore, that the blood 

 has a strong tendency to clot, rather than that it has no special tendency 

 thereto. 



But it has been recently suggested that the reason why blood does not 

 coagulate in the living vessels, is that the factors which are necessary for 

 the formation of fibrin are not in the exact state required for its produc- 

 tion, and that at any rate the fibrin ferment is not formed or is not free 

 in the living blood, but that it is produced (or set free) at the moment 



