248 MANUAL OF PHYSIOLOGY. 



will form around it. From this it would appear that the presence 

 of a substance which cannot carry on the necessary chemical in- 

 tercourse with the blood will excite irritation in its elements, and 

 so effect slight local death of the plasma and the production of 

 fibrin. 



The time required for the production of intravascular coagula- 

 tion as a result of mere stasis is happily long, for it has been fouud 

 that the blood current may be stopped in a limb, by pressure or 

 otherwise, for many hours without coagulation occurring. In- 

 deed, cases have occurred where a tight bandage has stopped the 

 circulation for an entire day without coagulation taking place. 

 This is explained by the fact that so long as the iiitima lives, the 

 blood remains fluid ; in short, the tissues die before the blood 

 clots in the vessels. 



The tissues continue to live for some time after an animal is 

 dead, and so we see the blood remains fluid in the vessels a long 

 time after death ; in fact, as long as the vessel wall can nourish 

 itself and live. Thus it has been shown that blood in a horse's 

 jugular vein separated by ligature from the circulation, and 

 removed from the animal, will remain fluid for fully twenty-four 

 hours. 



In cold-blooded animals the tissues live for even a longer 

 time. The heart of the tortoise, if kept under suitable conditions, 

 will beat for two days when removed from the body, and as 

 Briicke has shown, blood contained in it will remain fluid until 

 after the heart is dead. 



If the details of fibrin formation be followed within the blood- 

 vessels, it is found that the injured spot or foreign body first bo- 

 comes covered over with white corpuscles, around which threads 

 of fibrin appear attached to the rough surface. As more fibrin is 

 formed and the layer thickens, only a few cells can be seen in its 

 meshes, but a great number always exist on the surface of the new 

 fibrin, forming a layer between it and the blood. It is, moreover, 

 remarked that coagulation has some relation to the abundance of 

 white cells in all spontaneously coagulating fluids. The more 

 cells, the firmer the clot. In pathological exudations, also, and 

 those acute serous collections which coagulate on removal from 



