SEC. 2. THE CORPUSCLES OF THE BLOOD. 



The Red Corpuscles. 



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

 puscles. 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 IJL 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 profile 

 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 com- 

 posed 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 cor- 

 puscle seen by itself has a very faint 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 



