THE BLOOD. 



51 



but firm pressure down the finger will at once force out a drop 

 from the punctured spot. The blood must be drawn imme- 

 diately into the capillary pipette lest it coagulate. When the 

 pipette is full to the mark 2, its point should be rapidly wiped 

 clean of any blood adhering to the outside, and the contents at 

 once blown into the artificial serum in the cup, F. A little 

 suction back and forth clears the tube of any blood-corpuscles 

 which may have adhered to the glass within. Both tubes 

 should be carefully washed before being put away. 



The mixture is now to be thoroughly agitated with the glass 

 rod, and before it has time to settle, a drop is placed in the 

 middle of the cell on the slide, D, care being taken that the 

 drop is not large enough to touch any part of the circumference 

 of the cell. The covering glass, E, should at once be placed 

 upon the cell. Should the drop be too large, so that when the 

 thin cover is adjusted it spreads out too much, the glass should 

 be cleansed and the attempt made anew. Finally, a small drop 

 of water or saliva is applied to the edge of the covering glass, 

 under which it circulates around the top of the cell, serving to 

 hold the cover in place and pre- 

 vent evaporation. The slide is 

 then put in position and when 

 the corpuscles have all settled 

 to the bottom of the fluid, the 

 counting should begin. The 

 following detailed plan is then 

 given by Dr. Keyes : 



"It is better to count each 

 of the sixteen squares and write 

 down its number separately, 

 so that in counting the square 

 beneath it, should there be any 

 doubt about counting a given 

 corpuscle lying upon the line, a glance at the number recorded 

 for the square above may remove all doubt. Many corpuscles 

 will be found lying upon the outside lines bounding the large 

 square. I have adopted the rule of rejecting all those lying 

 upon the upper and right-hand outside lines (of the large 

 square) and counting all those lying on the lower and left- 

 hand outside lines. 



After having thus obtained the number of red corpuscles 



FIG. 22. Blood corpuscles as seen with the 

 squared ocular micrometer. (Keyes.) 



