204 
BLOOD-DISCS OF MAN AND MAMMALS. 
No nucleus can be distinguished in them, but they present a 
dark central spot, which is an optical effect of their bi-concave 
form; and this spot may be made to disappear by the addition 
Fig. 115.—Red Corpuscles of Human Blood. 
Seen separately at a , aa showing the front view, b the profile or edge view, and * a 
three-quarter view; at b united with each other so as to form columns like pibs 
of money; at c in a state of alteration such as exposure to air will produce; 
d shows a colourless corpuscle, or lymph-glohule. 
of water to the liquid in which they are suspended, the discs 
first becoming flat, then bulging-out on either side, and at 
last swelling so as to burst. The reason of this will be pre¬ 
sently explained* (§231). In Man and Mammals generally, 
the diameter of these blood-discs varies from about 1-2800th to 
1-4000th of an inch; but in the small Musk-deer , it is less 
than 1-12,000th. In the Camel tribe, the discs are oval, as 
in the lower Yertebrata. 
230. In Birds, Beptiles, and Fishes, the blood-particles 
present some curious differences from those of Mammalia. 
In the first place, they are much larger; their form, also, is 
oval instead of being round; and instead of being depressed 
in the centre, they bulge-out on each side. This bulging is 
Fig. 116 . —Blood Corpuscles of Pigeon. 
At a are seen the red corpuscles 6, and the colourless, or lymph globules c, c; at 
u, a red corpuscle treated with acetic acid; and at c, the same treated with water, 
so as to render the nucleus more distinct. 
evidently occasioned by the presence of a nucleus which is 
more solid than the rest; the nucleus, however, is not so well 
