TECHNIC (BLOOD AND BLOOD-FORMING ORGANS). 233 



Filter, and the solution will be ready for use. The white blood-cells 

 are stained violet, and may thus be counted with the red. 



The diluting fluid contained in the capillary tube is then blown out, 

 and a small drop of the diluted blood is placed on the center of the small 

 glass disc. The small disc is surrounded by a ring of glass, cemented to 

 the slide. This glass ring is o.i mm. thicker than the glass disc. When 

 this small chamber is covered with a thick cover-glass, we have a layer 

 of blood o. i mm. deep between the disc and the cover-glass. On the 

 upper surface of the small glass disc (on which the drop of diluted blood 

 was placed) there are marked off 400 small squares. The sides of the 

 small squares are ^V rnm. long. It will be seen that the layer of blood 

 over each of the squares would have a cubical contents of 



?oW of a cubic millimeter (^ X io X TO = 3^0 <y)- 



The hemocytometer slide is now placed on the stage of the micro- 

 scope, where it should remain undisturbed for several minutes before 

 counting. The red blood-cells in 25 to 50 squares are then counted. 

 To ascertain the number of red cells in a cubic millimeter the following 

 formula may be useful : 



(each mass of") ( ,., .. ") - f j n 1 f 



blood counted, 1 Xd \ t^' [ X" \ S F 

 rtWc.mm. j l hereICO J l counted j I number of red 



n (number of squares counted) ; 1 blood-cells in 



1 I c.nim. 



Or, ascertain the average of the red blood-cells in the squares 

 counted, and multiply this number by 400,000. 



In case it is desired to count only the white blood-corpuscles, a ^ per 

 cent, solution of glacial acetic acid is used for diluting the blood. This 

 solution bleaches the red cells, and brings out clearly the white corpuscles. 

 The blood is diluted only ten times, using for this purpose the Thoma- 

 Zeiss pipette for counting white corpuscles. The formula then reads as 

 follows : 



( the number of white "| 

 4000 X d(io) X n \ blood - corpuscles > 



( counted. J the number of white 



= blood-cells found in 



n (number of squares counted). a cubic millimeter. 



Or, multiply the average number of white corpuscles in each square 

 by 40,000. 



Lymph-glands. To obtain a general idea of the structure of 

 lymphatic glands, sections are made of small glands fixed in alcohol or 

 corrosive sublimate. They are then stained with hematoxylin and eosin. 

 In such preparations the cortical and medullary substances can be studied ; 

 the trabeculae and blood take the eosin stain. 



The flattened endothelial cells covering the trabeculae are brought 

 to view by injecting a o. i% solution of silver nitrate into a fresh lymph- 

 atic gland. After half an hour the gland is fixed with alcohol and car- 

 ried through in the regular way ; the sections should be quite thick (not 

 under 20 //). After the sections have been mounted in Canada balsam 

 and exposed to light for a short time, the endothelial mosaic will be seen 

 wherever the silver nitrate has penetrated. 



