134 SPECIAL PHYSIOLOGY 



zontal position in a safe place, should be rolled again for three minutes. 

 Fill the counter and adjust the cover-glass carefully as before; count 

 another group of thirty-six spaces and record the results obtained. 

 If the averages of the two counts differ more than one, the same 

 procedure must be carried out the third time, and the average of 

 the two fields nearest alike taken and the estimate made. 



To compute the number of corpuscles per cubic millimetre, find 

 the average number of cells for each space and multiply this by 

 4000, as each space is -^ mm. X 2T mm - X yV mm - This will give 

 the actual number of cells per cubic millimetre in the diluted blood. 

 Then make the correction for the dilution of the blood by multiplying 

 by 100 or 200, and the result will be the number of red cells per 

 cubic millimetre in the specimen of blood taken. In the example 

 given above it would work out as follows: 



First 36 spaces, 6^-; second 36 spaces, 6|-f . Average of the two 

 groups, 6V 9 g-. 6i X 4000 - 25,000 X 200=5,000,000, the number 

 of red cells per cubic millimetre. 



Precautions. The cement used on the counting slide is dissolved 

 by alcohol or ether; so these liquids should not be used on the plate. 

 Roll the filled pipette between the thumb and finger, and do not 

 shake the pipette, as some of the solution is sure to be lost. A com- 

 mon source of error that can easily be detected, but which is often 

 overlooked, is the unequal distribution of cells on the marked plate. 

 As soon as the drop is placed on the marked plate the cells begin 

 to settle, and, of course, most of them settle where the drop is thickest, 

 that is, in the center. This can be avoided by getting the cover- 

 glass in place quickly and making the whole drop of an even thickness. 

 Each specimen, before being counted, should be tested to see that the 

 corpuscles are evenly distributed over the whole drop. For the same 

 reason the filled counting slide should be kept in a horizontal position. 



Theoretically, counting the cells in one small space should be 

 sufficient, and it would be if the measurement and dilution of the 

 blood and distribution of the cells were all perfectly accurate. This 

 is impossible, and the errors are mostly eliminated by the methods 

 given. It is best for beginners always to make three counts of 36 

 spaces each from the same pipette and take the average. 



Questions. 1. Why is alcohol used to dry and not to clean the 

 pipette ? 



2. Why should the marked plate be dried without friction? 



3. What does hydrogen peroxide do to clean the pipette? 



4. Why rotate the needle while withdrawing it from the ear? 



5. Why wipe away the first drop of blood? 



6. Why wipe the end of the pipette before putting it into the 

 diluting solution? 



7. Why roll the pipette as the blood enters the bulb? 



8. Why blow out a few drops before putting a drop on the slide ? 



