28 INFLUENCE OF BLOOD VESSELS. [BOOK i. 



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 por- 

 tion 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 ligatured may be sus- 

 pended in a framework and opened at the top so as to imitate 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 pre- 

 pared, 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 spontane- 

 ously. If, so soon after death as the body is cold and the fat 

 is solidified, the pericardium be carefully removed from a sheep 

 by an incision round the base of the heart, the pericardial 

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

 little time after death, possesses the power of clotting) may be 

 kept in the pericardial bag as in a living cup for many hours 

 without clotting, and yet a small portion removed with a pipette 

 clots at once. 



This relation between the blood and the vascular wall may 

 be disturbed or overridden : clotting may take place or may be 

 induced within the living blood vessel. When the lining mem- 

 brane is injured, as when an artery or vein is sharply ligatured, 

 or when it is diseased, as for instance in aneurism, a clot is apt to 

 be formed at the injured or diseased spot ; and in certain morbid 

 conditions of the body clots are formed in various vascular tracts. 

 Absence of motion, which in shed blood, as we have seen, is un- 

 favourable to clotting, is apt within the body to lead to clotting. 

 Thus when an artery is ligatured, the blood in the tract of artery 

 on the cardiac side of the ligature, between the ligature and the 

 branch last given off by the artery, ceasing to share- in the circula- 

 tion, remains motionless or nearly so, and along this tract a clot 

 forms, firmest next to the ligature and ending near where the 

 branch is given off; this perhaps may be explained by the fact 

 that the walls of the tract suffer in their nutrition by the stagna- 

 tion of the blood, and that consequently the normal relation be- 

 tween them and the contained blood is disturbed. 



