150 



HANDBOOK OF PHYSIOLOGY. 



tents of the stem can be displaced unmixed we shall have in the mixture the 

 proper dilution. The blood and the saline solution are well mixed by shaking 

 the pipette, in the ball of which is contained a small glass bead for the pur- 

 pose of aiding the mixing. The other part of the instrument consists of a 

 glass slide (fig. 131) upon which is mounted a covered-disc, m, accurately ruled 

 so as to present one square millimetre divided into 400 squares of one-twentieth 



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Fig. 131. Thoma-Zeiss Hsemacytometer. 



of a millimetre each. The micrometer thus made is surrounded by another 

 annular cell, c, which has such a height as to make the cell project exactly 

 one-tenth millimetre beyond m. If a drop of the diluted blood be placed upon 

 m, and c be covered with a perfectly flat cover-glass, the volume of the diluted 

 blood above each of the squares of the micrometer, i.e., above each ^, will 

 be joW f a cubic millimetre. An average of ten or more 

 squares are then taken, and this number multiplied by 4000 X 100 

 gives the number of corpuscles in a cubic millimetre of un- 

 diluted blood. 



CHEMICAL COMPOSITION OF THE BLOOD. 



Before considering the chemical composition of the 

 blood as a whole, it will be convenient to take in order 

 the composition of the various chief factors which have 

 been set out in the table on p. 132, into which the blood 

 may be separated, viz. : (1.) The Plasma (2.) The 

 Serum ; (3.) The Corpuscles ; (4.) The Fibrin. 



(1.) The Plasma. The Plasma, or liquid part of 

 the blood, in which the corpuscles float, may be ob- 

 tained free from colored corpuscles in either of the ways 

 mentioned below. 



In it are the fibrin factors, inasmuch as when ex- 

 posed to the ordinary temperature of the air it under- 

 goes coagulation and splits up into fibrin and serum. 

 It differs from the serum in containing fibrinogen, but 

 in appearance and in reaction it closely resembles that 

 fluid; its alkalinity, however, is greater than that of the 

 serum obtained from it. It may be freed from white corpuscles by filtra- 

 tion at a temperature below 5 C. (41 F.) or by the centrifugal machine. 



The chief methods of obtaining plasma free from corpuscles may be here 

 epitomized : (1) by cold, the temperature should be about C. and may be 

 two or three degrees higher, but not lower. (2) The addition of neutral salts, 

 m certain proportions, either solid or in solution, e.g. of sodium sulphate, if 

 solid 1 part to 12 parts of blood ; if a saturated solution 1 part to 6 parts of 

 blood ; of magnesium sulphate, of a 23 or if saturated solution 1 part to 4 of 



Fig. 132. Thoma- 

 Zeiss Haemacyto- 

 meter. 



