FORENSIC MICROSCOPY. 120 
their recognition is not easy. In water, for example, 
the corpuscles swell up to a spherical form. The com- 
monest objects which may be mistaken for blood-cells 
are the spores of certain plants, and medico-legal litera- 
ture contains ludicrous instances of erroneous expert 
opinions based on such findings. With a good prepara- 
tion carefully compared with specimens of fresh blood, 
such errors can easily be avoided; and, since the cor- 
puscles of birds are of oval shape and show a distinct 
nucleus, blood-stains of avian origin may be excluded. 
The blood of other mammals can be differentiated from 
that of man only by the size of the cells, and the differ- 
ences are so slight that a certain result can seldom be 
‘reached. The blood of the goat has cells less than 4.5 pt 
in diameter, while the corpuscles of the horse and cow 
and pig are under 6 »; but the corpuscles of the dog, 
rabbit, mouse, cat, and other domestic animals very 
closely resemble those of human blood. Such differ- 
ences of one or two micromillimeters cannot be relied 
upon, since even in the fresh blood of a living animal 
some variation occurs and distorted dried specimens 
are much more dubious. Dr. E. L. Walker has pointed 
out that the white blood-cells of different mammals differ 
much more widely than do the red corpuscles; and the 
study of these bodies may furnish valuable aid to the 
microscopist. 
‘ The crucial test for blood-stains, and the only method 
by which the blood of man may be definitely distinguished 
from that of other mammals, depends upon a specific 
biologic reaction similar to that which is used in the 
