CHAPTER XVI. 



THE BLOOD (Cont'd). 



The Defensive Mechanisms of the Blood. 



The Coagulation of the Blood. Whenever a blood vessel is 

 slightly cut, the blood, which at first comes very freely, soon 

 ceases to flow because of the formation of a plug or clot of blood 

 at the site of the injury. The process by which the blood spon- 

 taneously forms the plug in the injured vessel is known as coagu- 

 lation, or clot formation. It protects the body from fatal hem- 

 orrhage in case of an ordinary wound. A clot is a semi-solid 

 mass, which on microscopical examination is seen to consist of a 

 meshwork of fibrils holding the blood corpuscles in their inter- 

 spaces. If blood is collected in a basin and whipped with some 

 twigs while it is clotting, the fibrils will collect on the twigs in 

 stringy masses, and the blood will remain fluid. The stringy 

 material is called fibrin. Obviously, fibrin cannot exist in the 

 blood stream, else the blood would form a clot within the blood 

 vessels; it is formed only when occasion demands, such as an in- 

 jury to the blood vessel. There are a number of experiments 

 which explain the process of coagulation. 



Thus, if blood is prevented from clotting by cooling it to 0. 

 centigrade, and is then mixed with a saturated solution of salt, 

 a white precipitate forms, which may be filtered off and dissolved 

 in 0.1 per cent salt water. This solution may be made to clot by 

 the addition of a very little blood from which the fibrin has been 

 removed. In other words, we have prepared a substance which 

 under proper conditions forms the fibrin of the clot. This sub- 

 stance is called fibrinogen, since it is the precurser of fibrin. 



Again, if blood be treated with sodium oxalate, it will not clot 

 unless calcium salts be added in amount sufficient to precipitate 



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