202 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



4. An emulsion of living leucocytes. To prepare this take 

 about 10 c.c. of normal saline solution containing J per cent, 

 of sodium citrate, to prevent the coagulation of the blood. 

 This must be freshly prepared (or kept sterile, which is incon- 

 venient), and the simplest method is to use "soloids" pre- 

 pared for the purpose by Messrs. Burroughs and Wellcome 

 (No. 2,456) ; one of these dissolved in 10 c.c. of water will yield 

 the solution required.* This is put into a centrifugalizing- 

 tube and warmed to blood-heat. A healthy person is then 

 pricked in the ear or finger, and his blood is allowed to drop 

 into the fluid until i c.c. or more has been collected. This is 

 then put into the centrifuge, very exactly counterbalanced, 

 and gently centrifugalized until all the corpuscles have come 

 to the bottom and the supernatant fluid is left clear, and no 

 longer. If the deposit is closely examined the red corpuscles 

 will be seen to be at the bottom, whilst above them there is a 

 thin whitish layer of leucocytes. Then, with a capillary 

 pipette armed with an indiarubber nipple, the whole of the 

 clear fluid is to be pipetted off as close as possible to the 

 leucocyte layer, but without disturbing the latter. The fluid 

 is replaced by normal saline, and the layer of corpuscles 

 gently mixed in, and the tube again centrifugalized for the 

 minimum time necessary to bring down the corpuscles. 

 Finally, the normal saline is pipetted off again, and the red 

 corpuscles and leucocytes thoroughly mixed together by 

 rolling the tube between the hands and tilting- it from side to 

 side. It seems to be important to avoid injury to the cor- 

 puscles as much as possible, so that it is inadvisable to cen- 

 trifugalize more violently or longer than necessary or to 

 mix reds and leucocytes by sucking them in and out of a 

 pipette. It is not necessary to attempt to concentrate the 

 leucocytes by taking only the top layer of the deposit. 



5. Two Wright's pipettes. These are drawn out from a 

 piece of ordinary glass tubing about 4 inches long* and about 

 as thick as a lead-pencil. This is held at each end, and the 

 central portion is thoroughly softened in a Bunsen or blow- 

 pipe flame, the tube being turned round the while. When 

 quite soft the tube is removed from the flame, and the ends 

 then pulled firmly and steadily apart until the softened portion 

 is pulled out into a thin tube (about the thickness of a steel 



* This very convenient method was suggested by Dr. Whitfield. 



