CHAP, i.] BLOOD. 27 



We might from this be inclined to conclude that blood clots 

 when shed but not before, because, fibrinogen being always present, 

 the shedding brings about changes which produce fibrin ferment, 

 not previously existing, and this acting on the fibrinogen gives rise 

 to fibrin. But we meet with the following difficulty. A very 

 considerable quantity of very active ferment may be injected into 

 the blood current of a living animal without necessarily producing 

 any clotting at all. Obviously, either blood within the blood 

 vessels does not contain fibrinogen as such, and the fibrinogen 

 detected by heating the blood to 56 C. is the result of changes 

 which have already ensued before that temperature is reached ; 

 or in the living circulation there are agencies at work which 

 prevent any ferment which may be introduced into the circula- 

 tion from producing its usual effects on fibrinogen ; or there are 

 agencies at work which destroy or do away with the fibrin, little 

 by little, as it is formed. 



22. And indeed when we reflect how complex blood is, and 

 of what many and great changes it is susceptible, we shall not 

 wonder that the question we are putting cannot be answered off 

 hand. 



The corpuscles with which blood is crowded are living structures, 

 and consequently are continually acting upon and being acted 

 upon by the plasma. The red corpuscles it is true are, as we shall 

 see, peculiar bodies, with a restricted life and a very specialized 

 work, and possibly their influence on the plasma is not very great ; 

 but we have reason to think that the relations between the white 

 corpuscles and the plasma are close and important. 



Then again the blood is not only acting upon and being acted 

 upon by the several tissues as it flows through the various 

 capillaries, but along the whole of its course, through the heart, 

 arteries, capillaries, and veins, is acting upon and being acted upon 

 by the vascular walls, which like the rest of the body .are alive, 

 and being alive are continually undergoing and promoting change. 



That relations of some kind, having a direct influence on the 

 clotting of blood, do exist between the blood and the vascular 

 walls is shewn by the following facts. 



After death, when all motion of the blood has ceased, the 

 blood remains for a long time fluid. It is not till some time 

 afterwards, at an epoch when post-mortem changes in the blood 

 and in the blood vessels have had time to develope themselves, 

 that clotting begins. Thus, some hours after death the blood in 

 the great veins may be found still perfectly fluid. Yet such blood 

 has not lost its power of clotting ; it still clots when removed 

 from the body, and clots too when received over mercury without 

 exposure to air, shewing that, though the blood, being highly 

 venous, is rich in carbonic acid and contains little or no oxygen, its 

 fluidity is not due to any excess of carbonic acid or absence of oxy- 

 gen. Eventually it does clot even within the vessels, but perhaps 



