104 PHYSIOLOGY CHAP. 



and the blood has been diluted 200 times, 1 c.mm. of blood will contain 



1 99 r, 



x 4000 x 200 = 3,920,000 red corpuscles. 

 250 



The white blood-corpuscles can be counted at the same time ; but if a 

 separate enumeration is desired for the sake of accuracy, the blood must be'' 

 agitated with a 0*3 per cent solution of acetic acid, in which the red corpuscles 

 will dissolve while the white remain intact. 



Hedin's Haematocrit is an apparatus for determining the total volume of 

 red corpuscles in 100 parts of blood. It consists of a small centrifuge (Fig. 25, 

 p. 98) and of two graduated tubes (a, ', Fig. 31). The determination is 

 carried out as follows : A small quantity of Miiller's fluid (sodium sulphate 1 

 grm., bichromate of potash 2 grins., distilled water 100 grins.) is taken up with 

 a pipette, and dropped into a small porcelain dish. The finger is then pricked 

 with a lancet so as to obtain a large drop of blood. With the same pipette a 

 quantity of blood equal to the quantity of M tiller's fluid is taken up, and 

 emptied into the same dish. The two fluids are then thoroughly mixed with 

 a glass rod, with the double object of retarding the coagulation of the blood 

 (the mixture will not clot under half an hour) and of fixing the red corpuscles 

 in their natural size. The graduated tube is then filled with the mixture 

 thus obtained, by taking up the fluid from the dish directly into the tube, 



Fin. 31. Hedin's Haematocrit substitute for test-tube holder of Fig. ~2'>, p. us. a, a', Graduated 

 tubes, kept in the hollows prepared for them by the presence of two elastic springs; 6, ?/, 

 small metal rods that compress the spirals in loosening or tightening the tubes. 



which has previously been fitted with a rubber tube furnished with a mouth- 

 piece. The tubes have a calibre of 1 sq. mm., and are. divided into 50 parts. 

 The filled tubes are then fixed to a horixontal holder (shown in Fig. 30), which 

 replaces the test-tube holders, pp, of the centrifuge (Fig. 25), care being taken 

 not to lose any of the fluid. They are now centrifuged for five to seven 

 minutes with a velocity of 80 turns per minute of the handle of the apparatus, 

 which corresponds to 8000 revolutions of the tubes, until the red corpuscles 

 separate out into a compact and well-marked column, the volume of which 

 will not shrink further. Since the tubes have very thick walls, and the 

 graduation is cut on the surface, errors may occur in the reading which are 

 due to the different positions of the reading eye. The inventor of the 

 apparatus has calculated this error as equal to 0'2 degree of the scale, and 

 to avoid it suggests that the reading be carried out by looking along a glass 

 plate held at right angles to the tube. From the volume found, the volume 

 of corpuscles in 100 parts of blood can be determined by multiplying the 

 volume of the column of blood-corpuscles by 4. As two determinations of 

 the volume of the erythrocytes are made at the same time, these form a 

 reciprocal control 



Since the total volume of the mass of corpuscles is proportional to the 

 relative number of the erythrocytes, this method can be substituted for that 

 of Thoma-Zeiss, which takes much longer to carry out. 



Many series of determinations on the relative mass of corpuscles 

 have been worked out, on man as well as animals, in order to 



