METHODS OF MAKING BI,OOD COUNTS. 



A general analysis of the blood is too elaborate to be introduced 

 bare, yet the following statement on making counts may be in- 

 teresting. Take a Thoma's hsematocytometer which consists of 

 a capillary tube graduated to two markings, dividing that end of 

 the tube into two equal halves, then a bulb having a capacity 

 ICO times that of the marked tube, and beyond the bulb a pro- 

 longation of the tube extended still further by a rubber tube for 

 aspiration. The blood is allowed to pass by capillary attraction 

 up to the first or second mark on the tube, the blood is then 

 wiped from its tip, the diluting fluid is then drawn in by aspira- 

 tion until the bulb is filled. It is mixed by shaking and is ready 

 for the counting chamber. The diluting fluid may be : Mercuric 

 chloride, 5 grams ; sodium sulphate, 5 grams ; sodium chloride, 

 I gram ; distilled water, 200 grams. The counting chamber is 

 made with a thick glass slide, on which is cemented a thin glass 

 plate with its central part cut out. The chamber thus formed 

 must be exactly -rirth of a millimeter deep. The floor of the 

 chamber is ruled in nine square millimetres, and the centre 

 square millimeter is subdivided into 400 small squares, each ^^ 

 square milUmeter. This centre millimeter alone is used in count- 

 ing the red globules, while the entire chamber is used in making 

 count of the leucocytes. The red cells counted in 100 of the 

 small squares must be multiplied by four to get the number in 

 the millimeter, then as the chamber is only y^ millimater deep, 

 it must be multiplied by 10 to get the number in the cubic milli- 

 meter, and finally as the blood was diluted 1:100, it must be 

 multipled by 100 to get the actual number in a cubic millimeter 

 of blood. Thus the count in 100 squares must be multiplied by 

 4,000 to give the actual number of red cells. If, on the con- 

 trary, the blood was only drawn up to the first marking on the 

 pipette instead of the second, the count must be multiplied by 

 8,000 instead of 4,000 to get the correct result. 



Great care is wanted in manipulation to get the requisite 

 amount of blood pure, in filling and covering the cell to avoid 

 air bubbles or overflow which would invalidate the count, and 



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