CH. XXVI.] 



THE BLOOD-PLATELETS. 



421 



deep, and the bottom of which is divided into one-tenth millimetre equares. 

 On the top of the cell the cover-glass rests. A standard saline solution 

 of sodium sulphate, or similar salt, of specific gravity 1025, is made, and 

 995 cubic millimetres are measured by means of the pipette into the glass 

 jar, and with this five cubic millimetres of blood, obtained by pricking 

 the finger with the needle, and measured in the capillary pipette (B), are 

 thoroughly mixed by the glass stirring-rod. A drop of this diluted blood is 

 then placed in the cell and covered with a cover-glass, which is fixed in 

 position by means of the two lateral springs. The layer of diluted blood 

 between the slide and cover-glass is J millimetre thick. The preparation is 

 then examined under a microscope with a power of about 400 diameters, 

 and focussed until the lines dividing the cell into squares are visible. 

 After a short delay, the red corpuscles which have sunk to the bottom of 



Fig. 352. nspmacytometer. (Cowers.) 



the cell, and are resting on the squares, are counted in ten squares, and the 

 number of white corpuscles noted. By adding together the numbers counted 

 in ten (one-tenth millimetre) squares, and, as the blood has been diluted, 

 multiplying by ten thousand, the number of corpuscles in one cubic milli- 

 metre of blood is obtained. The average number of corpuscles per cubic 

 millimetre of healthy blood, according to Vierordt and Welcker, is 5,000,000 

 in adult men, and 4,500,000 in women ; this corresponds to an average of 

 50 and 45 corpuscles respectively per square of Cowers' instrument. 



A haemacytometer of another form, and one that is much used at the 

 present time, is known as the Thoma-Zeiss hsemacytometer. It consists of a 

 carefully graduated pipette, in which the dilution of the blood is done ; this 

 is so formed that the capillary stem has a capacity equalling one-hundredth 

 of the bulb above it. If the blood is drawn up in the capillary tube to the 

 line marked 1 (fig. 354) the saline solution may afterwards be drawn up 

 the stem to the line 101 ; in this way we have 101 parts, of which the blood 

 forms i. As the contents of the stem can be displaced unmixed we ehal 



