Now, if you examine these ridges with a magnifying-glass, you will find arranged along each ridge a number of little dark spots.

Each of these points is the mouth of a very small tube.

This is called a _sweat duct_.

These ducts run down through both the outer and inner layers of the skin.

At the under side of the true skin the end of the tube is rolled up in a coil, as you can see by looking at the illustration on the following page.