CHAPTER XI 



METHODS OF PREVENTING OR CONTROLLING 

 HT:M0RRHAGE— HiEMOSTASIS. 



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

 from the contraction of the elastic fibres and muscular coats and the 

 mechanical closure of the bleeding orifice. The flow being checked a clot 

 soon forms, which seals the aperture, partly b}- its contraction, parti}' 

 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 \alve. 

 Bleeding is therefore often slight even when ver}- 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 3-et clear, and although we know that a thrombus 

 forms, yet the reason of its formation is still disputed. 



Briicke has shown that the movement of the blood and the influence 

 of the undamaged tunica intima are the two essential factors in pre- 

 serving the fluidity of the blood, but authorities differ as to the exact 

 mechanism by which thrombi are formed. It was formerly believed 

 that the blood simply clotted, as happens under the influence of the 

 fibrin ferment on the fibrinogen when blood is withdrawn from a 

 vessel. Later investigations have shown, however, that the thrombus 

 does not always consist of coagulated blood, and thrombi have been 



