PRACTICAL EXERCISES 59 



each square - f fa sq. mm. The volume of the column of liquid 

 standing upon a square is j^Vo curj - mm - One cu b- mm. of the 

 diluted blood would therefore contain 4,000 times as many corpuscles 

 as one square. But the blood has been diluted 100 times, there- 

 fore i cub. mm. of the undiluted blood would contain 400,000 times 

 the number of corpuscles in one square. Suppose the average for 

 a square is found to be 13. This would correspond to 5,200,000 

 corpuscles in i cub. mm. of blood. Compare your result with the 

 true number supplied by the demonstrator. (2) Prick the finger 

 to obtain a drop of blood, and repeat the count as in (i).* 



To Count the White Corpuscles. Add to i part of blood 9 parts 

 of ^ per cent, acetic acid, in order to lake the coloured corpuscles 

 and render it easy to see the leucocytes. 



8. Relative Volume of Corpuscles and Plasma by Haematocrite. 

 (i) For practice, fill the two graduated glass tubes with the de- 

 fibrinated blood of an animal. The rubber tube with mouthpiece 

 supplied with the apparatus is to be attached to the glass tube, and 

 the blood sucked up. Press the tip of the index-finger against the 

 pointed end, and carefully remove the rubber tube. Place the tube 



3 



FlG. 13. H^EMATOCRITE. 



A, haemotocrite attachment with graduated tubes ; B, automatic pipette for 

 filling the tubes (Daland). 



in the haematocrite frame, blunt end outwards that is, farthest 

 from the axis of rotation and then slip the pointed end down into 

 position against the spring. Instead of the rubber tube, a special 

 suction pipette for automatically filling the graduated tubes may be 

 employed (Daland). Attach the haematocrite frame to the centri- 

 fuge, and rotate till the volume of sediment (corpuscles) ceases to 

 diminish. The graduations are best read with a hand lens. The 

 leucocytes will be seen to form a thin whitish line proximal to the 

 column of red corpuscles. 



(2) Prick the finger or the lobe of the ear, fill the tubes as in (i), 

 and centrifugalize. Everything must be done as rapidly as possible, 

 so that the blood may not clot till the separation of plasma and cor- 

 puscles is completed. The centrifuge must rotate very rapidly 

 (about 10,000 revolutions a minute) for two or three minutes. For 

 clinical purposes it is best to rotate the centrifuge always at the 

 same speed for the same length of time rather than to aim at reaching 

 a constant length of the column of corpuscles. In this way useful 



* If the tube has not been properly filled, blow the blood out immedi- 

 ately. On no account permit it to clot in the capillary tube 



