BLOOD. 215 



To calculate the number of erythrocytes per cubic millimeter 

 of undiluted blood proceed as follows : Determine the number of 

 corpuscles in any given number of squares and divide this total by 

 the number of squares, thus obtaining the average number of 



FIG. 72. 



ORDINARY RULING OF THOMA-ZEISS COUNTING CHAMBER. (Da Costa.} 



erythrocytes per square. Multiply this average by 4,000 to obtain 

 the number of erythrocytes per cubic millimeter of diluted blood, 

 and multiply this product by 100 or 200, according to the dilution, 

 to obtain the number of erythrocytes per cubic millimeter of undi- 

 luted blood. Thus : 



Average number of erythro- , , Number of erythrocytes 



cytes per square per cubic millimeter. 



Great care should be taken to see that the capillary pipette is 

 properly cleaned. After using, it should be immediately rinsed out 

 with the diluting fluid, then with water, alcohol and ether in the 

 sequence given. Finally dry air should be drawn through the 

 capillary and a horse hair inserted to prevent the entrance of dust 

 particles. 



In counting leucocytes by means of the hsemocytometer proceed 

 as follows: As mentioned above, if the diluting fluid is either 

 Toison's or Sherrington's solution the leucocytes may be counted 

 in the same specimen of blood in which the erythrocytes are counted. 

 When this is done it is customary to use a slide provided with 

 Zappert's modified ruling (Fig. 73, p. 216). This method is rather 



