36 BLOOD. 



are agencies at work which prevent any ferment which may be introduced 

 into the' circulation from producing its usual effect on fibrinogeii ; 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 the many 

 and great changes of which it is susceptible, we shall not wonder that the 

 question we are putting cannot be offered 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 influ- 

 ence 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 im- 

 portant. 



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 pro- 

 moting 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 shown 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 until some time afterward, at an epoch 

 when post-mortem changes in the blood and in the bloodvessels have had 

 time to develop 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, showing that, though the blood, being highly venous, is rich in car- 

 bonic acid and contains little or no oxygen, its fluidity is not due to any 

 excess of carbonic acid or absence of oxygen. 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 ves- 

 sels. 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 excised, the blood 

 in many cases remains perfectly fluid, along the greater part of the length 

 of the piece, for twenty-four or even forty-eight hours. The piece so liga- 

 tured may be suspended in a framework and opened at the top so as to imi- 

 tate a living test-tube, and yet the blood will often remain long fluid, though 

 a portion removed at any time into a glass or other vessel will clot in a few 

 minutes. If two such living test-tubes be prepared, the blood may be 

 poured from one to the other without clotting taking place. 



A similar relation of the fluid to its containing living wall is seen in the 

 case of those serous fluids which clot spontaneously. If, as soon after death 

 as the body is cold and the fat is solidified, the pericardium be carefully re- 

 moved from a sheep by an incision round the base of the heart, the peri- 

 cardia! fluid (which, as we have already seen, during life and some little 



