NORMAL H^EMATOLOGY 



131 



red cells and estimating their number. First, to gain a clear idea 

 and understanding of the number of red cells in normal blood, and, 

 second, to be able readily and accurately to estimate the number of 

 red cells per cubic millimetre in any given clinical case. 



FIG. 67 



The haematocrit. The attachment at the upper end of the vertical shaft is made to rotate at 

 a speed of 7000 to 10,000 per minute by means of the gear-work of the body of the instrument. 

 Each arm of the rotating attachment is provided with a capillary tube which is graduated into 

 100 divisions. If the tube be filled with blood and rotated for two or three minutes at the 

 speed above mentioned the corpuscles will be thrown to the outer end and the volume per cent, 

 may be read off on the tube. B, an enlarged view of tube with centrifugalized blood. 



There are three methods of estimating the number of red cells 

 per cubic millimetre. 



1. The Thoma Haemacytometer. An instrument by which, with 

 accurate dilution, the corpuscles may be actually seen and counted 

 in a known space (Fig. 66). 



