138 THE BLOOD 



fresh blood from the tip of your finger under sterile conditions and mount 

 on a microscopic slide in a few drops of salt-solution, and examine immediately 

 under the high power. Small threads of fibrin will presently be seen to 

 form across the field, usually being most clearly obvious where fragments 

 of white corpuscles are noted, see figures 107 and 132. The threads of 

 fibrin become more apparent when stained with rosanilin. 



c. Whipped Blood. Draw a sample of blood into a glass tumbler, 

 enough to fill it one-half or two-thirds full. Immediately begin vigorously 

 stirring the blood with a bunch of stiff wires or a pencil, and keep it up until 

 the time of clotting has passed, 5 or 10 minutes. In this instance the wires 

 will break up and collect the fibrin as fast as it forms, and no firm mass will 

 be produced. The remaining fluid is called whipped blood. The fibrin can 

 be removed from the wires and washed in tap water until all the adherent 

 red corpuscles are removed. This mass of fibrin is white, elastic, and com- 

 posed of a network of thread-like fibers. It is these fibers extending through 

 and through the mass of blood which makes it retain the form of the vessel 

 when undisturbed clotting occurs. 



d. The Influence of Salt-Solution on Blood-Clotting. Add 20 c.c. of satu- 

 rated magnesium sulphate, i per cent sodium oxalate, and 2.5 per cent of 

 sodium chloride in each of 3 beakers. Draw into each beaker 50 to 60 c.c. of 

 blood and immediately mix thoroughly and let stand. The magnesium and 

 oxalate beakers will not coagulate even though they stand for days, but the 

 sodium-chloride blood will clot in a few minutes. 



The magnesium-sulphate blood will coagulate if diluted with a sufficient 

 amount of distilled water or physiological saline solution. Make a series 

 of dilutions and note when coagulation takes place. The sodium-oxalate 

 blood will coagulate when a sufficient excess (i per cent) of calcium chloride 

 is added to neutralize the excess of sodium oxalate. Demonstrate this on 

 a series of samples. 



If a liter or so of magnesium or oxalate blood is secured and separated 

 by a centrifuge, or by leaving stand for a sufficient time, a sample of salted 

 plasma will be obtained. This sample will coagulate when it is treated as 

 just described for salted blood, showing that the antecedents of fibrin are 

 found in the plasma. 



e. Action of Tissue Extracts on Coagulation. Wash out the blood of 

 a small animal by circulating 0.9 per cent saline through the arteries until 

 the outflowing fluid from the veins is clear. Take an organ, the liver for 

 example, grind it up in a sausage mill by running it through the mill two 

 or three times, then extract with 0.9 per cent physiological saline. The 

 macerating mass should be shaken up at intervals, and may be kept from 

 spoiling by adding an excess of chloroform or by keeping on ice. A few 

 cubic centimeters of this fluid extract added to a sample of freshly drawn 

 blood will very greatly hasten the rapidity of coagulation. This tissue ex- 



