132 SPECIAL PHYSIOLOGY 



2. The Oliver Haemacytometer. This instrument depends upon the 

 transmission of a transverse line of light from a candle through a 

 flat glass tube. The blood stops this light until a certain dilution 

 is obtained. The tube is graduated to read in the number of cells 

 per cubic millimetre of the blood used according to the dilution. 



3. The Hsematocrit. By this instrument is obtained the volume 

 of the corpuscular elements in the blood by centrifugation. From 

 this the number of red cells per cubic millimetre may be estimated 

 except in some special cases (Fig. 67). 



A. To Count the Red Blood Corpuscles. 



Appliances. Microscope with one-fifth-inch objective and me- 

 chanical stage; Thoma corpuscle counter, consisting of the ruled 

 counting slide and the diluting pipettes; glover's needle; three small 

 beakers and as many open dishes. 



Preparation. Wash the counting slide with water or soap and 

 water only when it needs it; the less it is handled the better. Usually 

 rinsing it in clean water and drying with a cloth is sufficient. Prepare 

 small beakers of distilled water and the diluting solution. Clean the 

 pipette as usual. 



Technique. Having prepared the apparatus and solutions, make 

 the puncture and fill the pipette by gently sucking a continuous 

 column of blood up to the mark "0.5" or "1" on the pipette, which 

 is near the bulb. Wipe the end of the pipette free from blood with 

 a clean cloth, but do not allow any blood to be drawn out by the 

 capillary attraction of the cloth. As soon as possible now insert the 

 point of the pipette into the diluting solution and suck up a con- 

 tinuous stream of solution until the mark "101" above the bulb is 

 reached. Roll the bulb between the thumb and finger as the blood 

 enters the bulb. 



If there is not blood enough to reach the mark "1," draw it only 

 to mark "0.5" and proceed in the same manner. Now hold the 

 pipette in the horizontal position with ends free and roll it back 

 and forth for three minutes to thoroughly mix the blood and solu- 

 tion. When thoroughly mixed blow out the contents of the cap- 

 illary below the bulb and then place a small drop on the marked 

 plate of the counting slide, putting just enough of the mixture on 

 it to fill the space between the marked plate and cover-glass, and 

 being careful not to allow any of the mixture to get into the moat. 

 Adjust the cover-glass over the drop quickly and carefully by placing 

 one edge of cover-glass in contact with the slide and letting the 

 opposite edge down gently with a needle. 



Place the counting slide when properly filled under the microscope 

 and find the upper left-hand corner of the marked area. Wait until 

 the corpuscles come to rest upon the surface of the marked plate, 



