28 INFLUENCE OF BLOOD VESSELS. [BOOK i. 



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 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, as 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 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 actu- 

 ally clotting under normal circumstances, may easily be made to 

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

 ting, is shewn by the fact that a foreign body, such as a needle 

 thrust into the interior of a blood vessel or a thread drawn through 

 and left in a blood vessel, is apt to become covered with fibrin. 

 Some influence exerted by the needle or thread, whatever may be the 

 character of that influence, is sufficient to determine a clotting, 

 which otherwise would not have taken place. 



The same instability of the blood as regards clotting is strikingly 

 shewn, in the case of the rabbit at least, by the result of injecting 

 into the blood vessels a small quantity of a solution of a peculiar 

 proteid prepared from certain structures such as the thymus body. 



