ESTIMATION OF THE RED CORPUSCLES 



201 



the dilution is read off directly from the figures on the lower 

 stem. 



The diluted blood thus obtained is spread out in a film of a 

 definite known thickness on the slide supplied on the instrument 

 (■^ig- 4-^! ")■ This is ruled in squares, and the squares are of 

 known size. The amount of blood lying upon each square is thus 

 known, and the number of corpuscles which lie upon it being 

 counted under the microscope, all the data for the calculation are 

 obtained. 



In blood examinations it is absolutely necessary that all points 

 in the technique should receive the most careful attention, or the 

 result will be worse than useless. For this reason we shall 

 describe each step in the process at some length, and advise the 



0100mm. 



C Zeiss 

 Jena. 



Fig. 41. — Thoma's H.emocytometer. 



practitioner to make several estimations before placing any 

 reliance whatever on his results. 



Requisites. — i. The hsemocytometer. 



2. A needle suitable for obtaining a small quantity of blood. 

 A straight Hagedorn's needle (about 2 inches long) is the very 

 best that can be used, and an ordinary hare-lip pin will answer 

 very well. It is best to use a needle with a flat cutting-point, and 

 not a round or triangular one, as the prick is less painful. A 

 piece of capillary tube drawn out in the flame, as already described, 

 is also very good and convenient. 



3. Diluting fluid. There are a good many formulae for this, 

 and some are rather complicated. Isotonic saline solution (com- 

 mon salt o-g or thereabouts) will answer perfectly ; it is advisable 

 to add to it a small quantity of some stain, methyl violet being 



