COAGULATION. 451 



Will be more or less bloody — that is, it will contain numerous red 

 corpuscles. If during the time of clotting the blood is vigorously 

 whipped with a bundle of fine rods, all the fibrin is deposited as a 

 stringy mass upon the whip, and the remaining liquid part con- 



Fig. 186.— Tho fihnn needles formed in tl,f clottin-; of blood. Plasma of oxalated 

 lilood clotted by throiiibm. The photof;i;ii)lis show ttie needles as seen with the idtranii- 

 eroscope. A, photographed by sun-light; B, by arc-liiiht. Only the needles lying in the focal 

 olane are seen distinctly. 



sists of serum plus the blood corpuscles. Blood that has been 

 whipped in tliis way is known as "defibrinated blood." It re- 

 sembles normal blood in appearance, but is different in its com- 

 position; it cannot clot again. The physiological value of clotting 

 is that it stops hemorrhages by closing the openings of the wounded 

 blood-vessels. 



Tirnc of Clotting. — In human blood the time necessary for clot- 

 ting varies greatly, according to the conditions to which the blood 

 is subjected after shedding. Blood allowed to flow from a wound 

 into a receiving vessel may clot within a few (3 to 10) minutes 

 according to the amount of blood drawn, the extent of surface 

 with which it comes into contact, the condition of the receiving 

 vessel, etc. The same blood taken from a vein into a clean 

 syringe, by the operation of venepuncture, and emptied into a 

 perfectly clean vessel, may require from 30 to 40 minutes before it 

 jellies throughout. If the surfaces with which the shed blood 

 comes into contact are coated with oil or vasehne, clotting may be 

 delayed for even a longer time. These differences may l)e under- 

 stood when we remember that clotting is a complicated process 



