Many animals that move about much in the dark, like cats and bats, for instance, have their lips or faces studded with long, delicate, stiff hairs called whiskers, which act in this way and prevent their bumping into objects in the dark.

And it is probable that the bristling of the hair on a dog's back, when he is angry or frightened, is in part for this purpose--to enable him to slip aside and dodge a blow, even after it has touched the ends of the hairs.

This great sensitiveness of the hair roots is what makes it hurt so when any one pulls your hair.
[20] See the diagram of the skin on page 171.
[21] You can easily test this by a very simple experiment.

Take a pair of dividers; or, if you haven't these, a couple of long pins or needles will do.

Set them with their points a quarter of an inch apart.