COUNTING THE BLOOD PLATES 9 



a larger drop of blood than for the red corpuscles. As the fluid 

 will run out of this larger pipette it is necessary to keep it as nearly 

 horizontal as possible. Acetic acid one per cent, (glacial one-third 

 per cent.) is preferable as the diluting fluid. With this fluid the 

 red corpuscles are made transparent, the nuclei of the leucocytes 

 standing out in bold relief. With a dilution of one part of blood 

 to ten of the acetic acid it is necessary to mix the blood and dilut- 

 ing fluid quickly; otherwise the leucocytes will be found in clumps, 

 making an accurate count impossible. The method of adjusting 

 the drop in the counting chamber is the same as for counting red 

 corpuscles. The leucocytes in the entire ruled area of the Thoma's 

 counting chamber, 400 squares, should be counted. In computing 

 the results multiply the number of leucocytes counted by 10, 

 since- the 400 squares represents .1 cmm. of diluted blood, and 

 multiply this product by 10, if the dilution was 1:10, to give the 

 number in undiluted blood. 



COUNTING THE BLOOD PLATES 



The blood plates show a marked tendency to undergo dissolu- 

 tion soon after the blood is taken from the blood vessels and to 

 adhere to each other and to foreign substances. Special precau- 

 tions must be taken in enumerating them. Several methods have 

 been used, three of which are given in detail. 



Pratt's method. Pratt used the following diluting fluid, 

 which keeps indefinitely unless moulds or bacteria develop: 



Sodium metaphosphate (Merck) 2 grams 



Sodium chloride 0.9 gm. 



Distilled water 100 cc. 



The number of erythrocytes is determined in the usual manner 

 with the Thoma hematocytometer. A few cubic centimeters of 

 the diluting fluid are placed in a watch glass. All the glassware 

 used must be perfectly clean. Blood is obtained from a puncture 

 free enough to allow the blood to flow freely. A sterilized platinum 

 loop, as used in bacteriological work, with a diameter of about 

 three mm. is filled with diluting fluid and the center of the loop 

 brought in contact with a fresh drop of blood. There should be 

 three or more parts of fluid to one of blood. A portion of the 

 mixture is at once placed on a slide and covered with a cover glass. 



