CHAP, i.] BLOOD. 27 



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 cor- 

 puscles 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, capil- 

 laries 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 

 never so firmly and completely as when shed. It clots first in the 

 larger vessels, but remains fluid in the smaller vessels for a very long 

 time, for many hours in fact, since in these the same bulk of blood 

 is exposed to the influence of, and reciprocally exerts an influence 

 on, a larger surface of the vascular walls than in the larger vessels. 

 And if it be urged that the result is here due to influences exerted 

 by the body at large, by the tissues as well as by the vascular walls, 

 this objection will not hold good against the following experiment. 



If the jugular vein of a large animal, such as an ox or horse, be 

 carefully ligatured when full of blood, and the ligatured portion 



