BLOOD. 



so treated is then filtered through a fine-mesh cloth. Defibrinated 

 blood will not clot spontaneously. 



II. THE NUMBER OF RED AND WHITE BLOOD CORPUSCLES. 



1. Counting the Erythrocytes or Red Corpuscles. The 



Thoma-Zeiss haemocytometer is used for this purpose. This con- 



FlG. 22. Thoma-Zeiss Haemocyto meter Counting-chamber, s, Glass slide, upon 

 which is mounted a covered disc m, accurately ruled to present one square millimetre 

 divided into 400 squares. This is surrounded by another annular cell, c, which pro- 

 jects in height exactly one tenth of a millimetre above m. 



sists of a graduated pipette for accurately diluting a known quan- 

 tity of blood with some fluid having the same osmotic pressure as 

 the blood. One of the most satisfactory diluents is physiological 

 salt solution. For human blood, this consists of a solution of so- 

 dium chlorid, 8.5 grams in 1000 c.c. of water. 



The capillary stem of the pipette, used for diluting the blood for 

 counting the red corpuscles, has a capacity equalling one-hun- 

 dredth of the hollow ball with which it joins (see Fig. 23). If the 

 blood is drawn up to the line marked i on the pipette stem and 

 then the diluent drawn in until the mixture reaches the line 101 



FIG. 23. Thoma-Zeiss Haemocytometer Pipette. 



marked just above the ball (see Fig. 23), there will be 101 parts of 

 fluid, of which the blood forms i. The contents of the stem, how- 

 ever, do not have to be considered after the dilution is made, since 

 they can be displaced, unmixed. The dilution of the blood will 

 then be i to 100. As the blood and diluting fluid enter the mixing 



[59] 



