THE BLOOD. 



149 



fifth of a millimetre deep, and the bottom of which is divided into one-tenth 

 millimetre squares. On the top of the cell rests the cover-glass, which is kept 

 in its place by the pressure of two springs proceeding from the stage plate 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 a 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 be- 

 tween the slide and cover-glass is inch thick. The preparation is then 

 examined under a microscope with a power of about 400 diameters, and f ocussed 

 until the lines dividing the cell into squares are visible. 



Fig. 130. Haemacytometer. (Gowers.) 



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

 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, mul- 

 tiplying by ten thousand, the number of corpuscles in one cubic millimetre of 

 blood is obtained. The average number of corpuscles per each cubic millimetre 

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

 men, and 4,500,000 in women. 



A hsemacytometer 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 ball 

 above it. If the blood is drawn up in the capillary tube to the line marked 1 

 (fig. 132) the saline solution may afterward be drawn up the stem to the line 

 101 ; in this way we have 101 parts of which the blood forms 1. As the con- 



