176 



PHYSIOLOGY. 



piece. With the blood up to the 1.0 mark and enough diluting 

 saline to bring the whole quantity of liquid to 101, the dilution is 

 1 to 100. 



The second portion of the instrument, known as the counting 

 chamber, is constructed so as to enable one to count under the micro- 

 scope all the cells in a known bulk of the diluted blood. In the 

 center of a thick glass slide is cemented a cover-glass of accurately 



Fig. 51. Daland's Hsematocrit. 



measured thickness with a hole in the center of about 1 centimeter 

 in diameter. In the central area of this cover-glass there is also 

 cemented to the glass slide a glass disc about 2 millimeters smaller 

 in diameter and exactly /lo millimeter thinner than the cover-glass. 

 The glass shelf being exactly %o millimeter thinner than the cover- 

 glass, it will readily be seen that if a second loose cover-glass be 

 laid upon the first, the under surface of this loose cover-glass will 



Fig. 52. Red Blood-corpuscles. (LANDOIS.) 



a, 6, Normal human red corpuscles with the central depression more or 

 less in focus, c, d, e, Mulberry forms, g, li, Crenated corpuscles, k, Pale 

 decolored corpuscles, i, Stroma. /, Frog's corpuscles acted upon by a strong 

 saline solution. 



be exactly % millimeter above the upper surface of the glass disc. 

 In this way there is secured a layer of fluid % millimeter in depth. 

 Furthermore, 1 square millimeter of the surface of the disc is out- 

 lined and subdivided by intersecting lines into 400 small squares. 

 For convenience in counting, every fifth row of squares is divided 

 into two by an additional line. The volume of diluted blood above 

 each square of the micrometer will be %ooo cubic millimeter. The 

 average of 10 or more squares is then ascertained, which result is 



