THE BLOOD. 73 



opposed to one another, which bear upon it, and these may be, for the 

 sake of clearness, classed under two heads: 



(1) Blood iv ill coagulate within the living body under certain condi- 

 tions, for example, on ligaturing an artery, whereby the inner and mid- 

 dle coats are generally ruptured, a clot will form within it, or by passing 

 a needle through the coats of the vessel into the blood stream a clot will 

 gradually form upon it. Other foreign bodies, e.g. wire, thread, etc., 

 produce the same effect. It is a well-known fact that small clots are apt 

 to form upon the roughened edges of the valves of the heart when the 

 roughness has been produced by inflammation, as in endocarditis, and it 

 is also equally true that aneurisms of arteries are sometimes spontaneously 

 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 re-agent, 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 oc- 

 curred 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 some time 

 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 fluidity under the usual 

 conditions of the blood within the living body must reconcile the above 

 apparently contradictory facts, and must at the same time be made to in- 

 clude all the other known facts concerning the coagulation of the blood. 

 "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 coagula- 

 tion out of the body is due to the action of foreign matter with which it 

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



