l6o THE BLOOD 



accurately ruled so as to present one square millimeter divided into 400 

 squares of one-twentieth of a millimeter each. In the older instruments 

 the rulings are on the cover disc. The micrometer surface is ground 

 below the general surface exactly one-tenth millimeter. If a drop of the 

 diluted blood be placed upon the ruled surface, and covered with a per- 

 fectly flat cover-glass, the volume of the diluted blood above each of 

 the squares of the micrometer, i.e., above each 3^oo square millimeter area 

 will be J^ooo f a cubic millimeter. An average of ten or more squares is 

 then counted, and this number multiplied by 4000 X 100 gives the number 

 of corpuscles in a cubic millimeter of undiluted blood. A separate pipet 

 is used for making dilutions for counts of leucocytes. In this, the dilution 

 is made of one part of blood and ten parts of diluting fluid. Acetic acid, 

 0.2 of one per cent., is usually employed for this purpose. 



5. The Percentage of Corpuscles and Plasma in Human Blood. 

 Fill the two graduated capillary tubes of a hematocrite with blood drawn 

 from the tip of your own finger, insert into the instrument, and centrifuge 

 as rapidly as possible. The experiment must be performed within the time 

 limit of clotting in order to be successful. The corpuscles will be thrown 

 down and the percentage of plasma and corpuscles can be read off directly. 

 Should one fail to fill the tube exactly full, then the percentage of plasma 

 and corpuscles can be calculated from the proportion which each bears to 

 the quantity in the tube. 



6. Estimation of the Percentage of Hemoglobin. The percentage 

 of hemoglobin in a sample of blood can best be obtained by either the Dare 

 or the Sahli hemoglobinometer. The principle of the Dare is given in the 

 text. It rests on a comparison of the color of a drop of undiluted blood 

 with a standard color scale when illuminated by candle light. Sterilize 

 a lobe of the ear or finger tip, draw a drop of blood, bring the edge of the 

 Ware blood pipette carefully against the blood drop, allowing it to flow 

 between the plates. Place this in the instrument and examine either in a 

 dark room or with the instrument turned toward a dark wall or paper. 



A sample of blood may be taken by a Miescher pipette diluted in the 

 Miescher hemoglobinometer, and its percentage of hemoglobin computed. 

 See text, page 140, for diagram of instrument and description of the 

 method. 



Perhaps a more convenient and certainly a quicker method for deter- 

 mining the percentage of hemoglobin is Talquist's hemoglobinometer. By 

 this method a drop of blood is drawn directly on to absorbent-paper furnished 

 with the instrument, and the resulting stain is compared directly with a paper 

 color scale which is graduated in percentage. In this method the comparison 

 is made in ordinary daylight, and because of its rapidity it is very convenient 

 for clinical examinations, though it is less accurate. 



