ESTIMATION OF THE RED CORPUSCLES 237 



and advise the practitioner to make several estimations before 

 placing any reliance whatever on his results. 

 Requisites. The haemocytometer. 



2. A needle suitable for obtaining a small quantity of 

 blood. A straight Hag'edorn'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 con- 

 venient. (See p. 37.) 



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

 this, and some are rather complicated. Isotonic saline solu- 

 tion (common salt 0*9 or thereabouts) will answer perfectly; 

 it is advisable to add to it a small quantity of some stain, 

 methyl violet being the best, although gentian violet will do 

 very well. This colours the leucocytes, so that they are 

 readily distinguished from the red corpuscles.* 



4. A microscope having a J-inch lens which will focus 

 through the thick cover-glass supplied with the hsemocyto- 

 meter. If the examination is not to be made by the bedside, 

 a strong indiarubber band a little shorter than the pipette 

 should be carried. 



PROCESS. 



i. Pricking the Patient. The blood may be procured from 

 the convex border of the lobe of the ear or from the lateral 

 surface of the last phalanx of the finger. The advantage of 

 the former situation is that the pain is very slight, the skin 

 being thin, and that the patient cannot see what you are 

 doing, and is not likely to start at the critical moment. It is 

 to be recommended for children and nervous women. The 

 advantage of the finger is that the skin is free from hairs, 

 and these are objectionable in the preparation of films by 

 the cover-glass method; an additional advantage is that the 

 patient can put his hand into the position most convenient 

 to you, and you have not to lean over him. 



The area of the skin to be punctured may be washed with 



* The following formula is better : Distilled water, 160 c.c. ; glycerin, 

 30 c.c. ; sodium sulphate, 8 grammes ; sodium chloride, i gramme ; methyl 

 violet, a trace (Toison's fluid). 



