HiEMOSTAStS. 115 



VII.— METHODS OF PREVENTING OR CONTROLLING 

 HEMORRHAGE— HJEMOSTASIS. 



The first check to the flow of blood from a divided vessel results 

 from the contraction of the internal and middle coats and the 

 mechanical closure of the bleeding orifice. The flow being checked 

 a clot soon forms, which seals the aperture, partly by its contraction, 

 partly by acting as a plug or cap. The slower the flow the more 

 quickly does such a clot form. The cardiac weakness following on 

 loss of large quantities of blood therefore favours cessation of bleeding. 



In incised wounds, the divided ends of the vessel retract into the 

 tissue, which then closes over them and checks haemorrhage. The 

 outer coat of torn and crushed vessels often becomes twisted into a 

 kind of cord, while the intima is induplicated after the fashion of a 

 valve. Bleeding is therefore often slight even when very large 

 vessels have been torn or crushed. A clot having once formed 

 extends, as a rule, up to the next collateral branch of the injured 

 vessel. 



The exact nature of the changes by which a divided vessel is 

 closed is not yet clear, and although we know that a thrombus forms, 

 yet the reason of its formation is still disputed. 



The thrombus either becomes replaced by connective tissue or it- 

 softens and breaks down. The former is the more favourable 

 development, and consists in the gradual formation of connective 

 tissue, which usually results from proliferation of the endothelial 

 lining of the vessel. The endothelium is first converted into poly- 

 morphous " formative cells," which rapidly multiply, extend into 

 the thrombus and replace it, leaving only a pigment residue (oxy- 

 hydrate of iron) of the original thrombus. From the vasa-vasorum 

 new blood-vessels extend into the new tissue, converting it practically 

 into connective tissue. The thrombus thus plays a passive part, 

 being gradually replaced by fibrous connective tissue. These changes 

 occur with comparative rapidity ; vascular new tissue may be found 

 at the point of ligation eight days after operation. Where, however, 

 the intima is extensively diseased, or where general constitutional 

 disturbance exists, the changes proceed more slowly. 



Circulation is usually restored by collateral paths, the smaller 

 vessels enlarging and conveying the blood by parallel channels. This 

 change also is often very rapidly effected, as shown by Nothnagel's 

 experiments on rabbits. Six days after ligation of an important 

 vessel the muscular coats of the dilated collateral vessels were found 



