COAGULATION OF BLOOD. 295 



from the vessel it is fluid, but at the end of two or three minutes its 

 fluidity is' so much diminished that it will not flow ; this consis- 

 tency increases until at the end of eight or ten minutes the entire 

 quantity of blood becomes a mass resembling currant-jelly in color 

 and consistency. This jelly-like mass becomes more and more 

 consistent, squeezing out upon its surface a few drops of a straw- 

 colored fluid the serum. As the shrinking of this gelatinous 

 mass the dot continues, it separates from the sides of the vessel 

 in which the blood was received, and the serum is squeezed out on 

 all sides, until at length there is a more or less solid clot floating 

 in a considerable quantity of serum. The entire process requires 

 from ten to forty -eight hours. When examined, the clot is found 

 to be made up of fibrin and corpuscles, the red corpuscles giving 

 to it the red color. The white corpuscles may at first be entangled 

 in the meshes of the fibrin, but by virtue of their ameboid move- 

 ment they soon escape into the serum. The serum has the same 

 composition as the plasma minus the fibrinogen. 



Although the corpuscles are denser than the plasma, still the 

 difference is so slight and the process of coagulation so rapid that 

 before they can settle they are entangled in the meshes of the 

 fibrin as it forms, and thus become a part of the clot. If any- 

 thing occurs to delay coagulation, the corpuscles settle, and the clot 

 is then less red and more yellowish. This delay may be brought 

 about by the addition of a 27 per cent, solution of magnesium 

 sulphate or other neutral salt, the plasma being then termed 

 salted plasma ; it occurs also in inflammatory processes, and hence 

 in the olden time, when venesection or " bleeding " was commonly 

 practised, this crusta phlogistica, or " buffy coat," was always looked 

 for by the physician, and when it formed was considered as evidence 

 that the bleeding was justifiable. That the physicians of that period 

 were not always right in this judgment is now known, for a buffy 

 coat will form in blood which is hydremic, a condition in which 

 bleeding is contraindicated. In horses 7 blood, which normally 

 coagulates very slowly, this " buffy coat" always forms. It is 

 simply the fibrin without the corpuscles, or at least without enough 

 of them to give the red color which the clot usually possesses. 



Influences which Retard Coagulation. Coagulation is retarded 

 by cold, by solutions of sodium or magnesium sulphate, by a 

 diminished amount of oxygen, by an increased amount of carbon 

 dioxid, by acids or alkalies, by egg-albumin, by oil, by a solution 

 of albumose, and by extract of the head of the leech. It is well 

 known that the blood drawn from the vessels by this animal does 

 not coagulate within its body. It is supposed that its saliva contains 

 an albumose which prevents clotting of the blood. Solutions 

 of potassium or sodium oxalate also prevent coagulation. The 

 explanation of this action is that the calcium which is required for 

 the process is precipitated as calcium oxalate. Venous blood 



