SEC. 2. THE CORPUSCLES OF THE BLOOD. 



The Red Corpuscles. 



24. The redness of blood is due exclusively to the red 

 corpuscles. The plasma as seen in thin layers within the living 

 blood vessels appears colourless, as does also a thin layer of serum ; 

 but a thick layer of serum (and probably of plasma) has a faint 

 yellowish tinge due as we have said to the presence of a small 

 quantity of a special pigment. 



The corpuscles appear under the microscope as fairly homo- 

 geneous, imperfectly translucent biconcave discs with a diameter of 

 7 to 8 p and a thickness of 1 to 2 //,. Being discs they are circular 

 in outline when seen on the flat, but rod-shaped when seen in pro- 

 file as they are turning over. Being biconcave, with a thicker 

 rounded rim surrounding a thinner centre, the rays of light in 

 passing through them, when they are examined by transmitted 

 light, are more refracted at the rim than in the centre. The effect 

 of this is that, when viewed at what may be considered the proper 

 focus, the centre of a corpuscle appears clear, while a slight opacity 

 marks out indistinctly the inner margin of the thicker rim, whereas, 

 when the focus is shifted either up or down, the centre becomes ) 

 dark and the rest of the corpuscle clear. Any body of the same 

 shape, and composed of substance of the same refractive power, 

 would produce the same optical effects. Otherwise the corpuscle 

 appears homogeneous without distinction of parts and without 

 a nucleus. A single corpuscle seen by itself has a very faint S 

 colour, looking yellow rather than red, but when several corpuscles 

 lie one upon the top of the other the mass is distinctly red. 



The red corpuscle is elastic, in the sense that it may be deformed 

 by pressure or traction, but when the pressure or traction is re- 

 moved regains its previous form. Its shape is also much influenced 

 by the physical conditions of the plasma, serum, or fluid in which 

 for the time being it is. If the plasma or serum be diluted with 

 water, the disc, absorbing water, swells up into a sphere, becoming 



