152 UNCONSCIOUS NERVOUS OPERATIONS 



drawn from the blood vessels, about one fourth its volume of a satu- 

 rated solution of sulphate of magnesia. This " salted " blood may be 

 kept in a cool place for several days without clotting. It may be 

 made to clot by diluting it with five to ten times its volume of water. 



A more satisfactory method of preventing the clotting of blood, the 

 writer has found, consists in adding oxalate of potash in the propor- 

 tion of one part of a 5 per cent solution of oxalate of potash to 

 twenty-five parts of blood. The oxalate solution of the requisite 

 amount should be placed in a vessel and the blood be allowed to flow 

 into and mix with it. The mixing should be made thorough by 

 vigorous shaking. To produce a clot, add a few drops of a 2 per cent 

 solution of calcium chloride to some of the oxalate-blood. The potas- 

 sium oxalate prevents clotting by precipitating the calcium salts nec- 

 essary to coagulation. The addition of calcium chloride restores the 

 calcium and renders clotting possible. 



74. The Minute Structure of the Fibrin Framework. To a drop of 

 fresh blood on a slide add two drops of normal salt solution. Put on a 

 cover glass and set aside an hour or so to clot. Add 50 per cent alco- 

 hol at the edge of the cover glass (to wash out the corpuscles and 

 harden the fibrin). Observe with the microscope the network of 

 fibrin fibrils. Care must be taken not to move the cover glass during 

 the preparation for examination. 



