NORMAL H^MATOLOOY 135 



9. Why draw air through the pipette after the ether is drawn out? 



10. What kind of a solution should be used to dilute the blood; 

 that is, what properties should it have? 



11. Why are there 101 parts in the pipette instead of 100? 



12. Is there any appreciable variation in the number of red cells 

 in normal individuals? 



13. If there is a variation, give some of the reasons. 



14. Account for the variations observed in members of your 

 section. 



B. To Count the White Blood Corpuscles. 



In counting the number of white cells per cubic millimetre in the 

 blood, the principle of diluting and counting is exactly the same as 

 in counting the red cells. There are some differences in the details, 

 since we must first get rid of the red cells so that the white cells can 

 be seen, and because of the small number we must dilute less and 

 count larger areas. 



Appliances. The instruments are the same, with two exceptions 

 or modifications. The diluting pipette is just like the red-cell diluting 

 pipette, except that it is larger in the capillary and smaller in the 

 bulb, so it can make dilutions of ten to one hundred instead of one to 

 one hundred. The counting plate should have the modified mark- 

 ings as shown in Fig. 68. One can use the lower power (f ) objective 

 of the microscope just as well and save time by it, but it is not 

 necessary. 



Reagents. The same as when counting the red cells except that 

 a different diluting solution must be used. A J per cent, solution of 

 acetic acid in distilled water will destroy the red cells and render them 

 invisible, while it will not destroy the white cells, but make them show 

 more plainly. 



WHITE-CELL DILUTING SOLUTION. 



Acetic acid 0.5 c.c. 



Distilled water q. s. ad 100.0 c.c. 



Technique. The technique of obtaining the blood and filling the 

 pipette is the same as with the red cells, except that the capillary tube 

 is so large that we must have more blood. For this reason for begin- 

 ners we recommend that only a half-part of blood be taken at first. 

 More accurate results can be obtained by using one part, but much 

 time and practice is necessary to fill the tube easily. The capillary 

 is so large that solutions will not stay in it, but run out quickly when 

 the tube is out of the horizontal position. First, then, we must have 

 more blood; usually two or three good-sized drops are sufficient. 

 Second, the tube must be held horizontal or the blood and solution 

 will run out. As the capillary tube is large it is very easily cleaned 

 and dried. 



