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 portion 

 excised, the blood in many cases remains perfectly jluid, along the 

 greater part of the length of the piece, for twenty-four or even 

 forty-eight hours. The piece so ligatured may be suspended 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 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, 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 hoars 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 overriden : clotting may take place or may be induced 

 within the living blood vessel. When the lining membrane 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 unfavourable 

 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 circulation, 

 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 stagnation of the 

 blood, and that consequently the normal relation between them 

 and the contained blood is disturbed. 



That the blood within the living blood vessels, though not 

 actually clotting under normal circumstances, may easily be made 

 to clot, that the blood is in fact so to speak always on the point 



