. Here is, at once, a sure way
of differentiating mammalian blood from that of the other three great
classes of animals. The only difficulty is that blood corpuscles get out
of shape, under certain circumstances, and are no longer either oval or
round. But there is another difference. A mammalian corpuscle is of
uniform substance throughout: that of a fish, bird, or reptile has a
small, dense spot near the centre, called a nucleus. Snails, slugs,
worms, and other low forms of animal life do not come into the question
at all, for their blood is generally colourless, and, if not, it is
blue-green, violet, brown, being scarcely ever red, and then not from
the presence of corpuscles.

All that remains for the analyst, therefore, supposing he finds a round
corpuscle, is to say to what mammalian animal it belongs. (The llama,
alpaca, camel, and their kin, by the way, have oval corpuscles.)

How are the corpuscles of different mammalia to be distinguished under
the microscope? Merely by their size. They have all been measured with
the greatest care, a specially small unit of length, called a micron,
having been invented for the purpose. It is only 1/25000 of an inch
long, and, expressed in tenths of a micron, the average diameter of a
human blood corpuscle is 77; of a dog, 73; of a rabbit, 69; of a cat,
65; of a sheep, 50; of a goat, 41; and of an elephant, 94. But these are
average measurements, and some corpuscles are smaller, some larger.

[Illustration: MORE TINY CLUES.--HUMAN HAIR CONTRASTED WITH ANIMALS'
HAIR, WOOL, AND FIBRE.

Cat's Hair. Bat's Hair. Berlin Wool. Reindeer's Hair. Woody Fibre.
Human Hair. Fox Hair. Hare's Hair. Squirrel's Hair. Human Hair Bulb.]

Therefore, when it is a question of whether the blood is that of a dog,
pig, hare, rabbit, or man, he would be a daring man that would give a
decided opinion. But it is certainly possible to come to a safe
conclusion as to whether it is that of a human being or a sheep, goat,
or elephant.

Owing to the influence of disease on the blood, however, it is never
really safe to say absolutely "This is human blood," and, in fact, all
that is generally stated in evidence is whether it is mammalian.

There is one other important piece of work the medical detective can
perform in his laboratory, in the way of tracking criminals; that is to
distinguish hairs from vegetable fibres, and human hairs from animals'.
Our illustrations show how it is do