102 



, oV ,, : : TEE 'BLOOD. 



water is added it passes in by diffusion, and the corpus- 

 cle swells and may burst. Likewise by diffusion when 

 they are placed in a liquid and it contains more salts than 

 the blood, water passes out and the corpuscle becomes 

 smaller and shriveled in appearance. In order to dilute 

 the blood without changing the size a solution containing 

 0.5 to 0.6 per cent, of sodium chlorid can be used. Such 

 a solution is called isotonic; that is, its osmotic pressure 

 is the same as that of the fluid within the corpuscles. 



Freshly-drawn blood, when allowed to stand undis- 

 turbed, in a few minutes becomes thickened to a dark- 

 red gelatinous mass. If the coagulation is slow the red 

 corpuscles have time to sink and collect with the fibrin 

 in a mass in the lower part of the vessel. The serum is 

 squeezed out of the mass and surrounds it, above and at 

 the sides. If the blood is beaten during the time of co- 

 agulation the fibrin does not separate as a gelatinous 

 substance, but in stringy masses, which have a high de- 

 gree of elasticity. The coagulation can be prevented or 

 hindered by cold and by the addition of neutral salts, 

 peptones, and some other substances. 



To determine the number of red corpuscles the apparatus of 

 Thoma-Zeiss may be employed. This consists of two pieces: a 

 pipette for measuring and diluting the blood and a cell for count- 

 ing the number with the aid of a microscope. The lower part of 

 the pipette is a graduated capillary tube for measuring the blood. 

 Above is a bulb which, being filled to the mark with the diluting 

 fluid, dilutes 200 times the blood which was measured in the capil- 

 lary tube. The counting-cell when covered with a cover-glass gives 

 a layer of blood 0.1 millimeter in depth. On the bottom of the cell 

 are ruled sixteen squares, each Vwo of a square millimeter in area. 

 They are surrounded by two rows of smaller rectangles. The 

 volume of blood over each of these squares, then, must contain 

 1 /4ooo cubic millimeter. If the number of corpuscles which are con- 

 tained in this V^ cubic millimeter is determined, the number in 



