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PHYSIOLOGY FOR DENTAL STUDENTS. 



other, ten times with 0.337% acetic acid. The former dilution is 

 for counting red, and the latter, for counting white corpuscles. 

 A drop of the diluted blood is then placed on a special glass 

 slide which contains a counting chamber of such a depth that 

 when a cover slip is put over a drop of fluid in the chamber, a 

 column of fluid one-tenth of a millimetre deep is obtained (Fig. 

 13). The chamber is graduated with cross lines, so that each 

 square represents a known fraction of a millimetre. The average 

 number of corpuscles found in a number of squares, by actual 

 count with a microscope, is multiplied by the factors of dilution 

 employed, the product being the number of cells in a cubic milli- 



Fig. 13. Thoma-Zeiss Haemocytometer ; M, mouthpiece of tube (G), by 

 which blood is sucked into S; B, bead for mixing; a, view of slide from 

 above ; b, in section ; c, squares in middle of B, as seen under microscope. 



metre of blood. The erythrocytes, which in health number about 

 five million in a cubic millimetre, may decrease to less than a 

 million in disease, such as pernicious anosmia, or after haemor- 

 rhage. On the other hand, they may number six or seven million 

 in people who live at high altitudes. The oxygen-carrying power 

 of the blood is proportional to the percentage of haemoglobin, so 

 that by estimating this and the number of corpuscles, a fair idea 

 of the condition of the blood is obtained. 



THE ORIGIN OF ERYTHROCYTES. It is interesting to inquire 

 into the source of the blood cells, but although this has been the 

 subject of many researches, it is by no means definitely settled 



