They are hardest and most solid on their surfaces, and hollow, or spongy, inside.

The long bones of the limbs are hollow, and the cavity is filled with a delicate fat called _marrow_--just as an elderberry stem or willow-twig is filled with pith.

This tubular shape makes them as strong as if they were solid, and much lighter.[25] The short, square, and flattened bones of the body, such as those of the wrist, the skull, and the hips, instead of being hollow inside are spongy; and the spaces in the bone-sponge are filled with a soft tissue called the _red marrow_ in which new red and white corpuscles for the blood are born, to take the place of those which die and go to pieces.
FOOTNOTES: [24] You can easily prove that a bone is made up of living tissue soaked and stiffened with lime, by putting it into a jar filled with weak acid.
This will gradually dissolve and melt out the lime salts, and then you will find that the bone has lost three-fourths of its weight and that what remains of it is so soft and flexible that it can be bent, or even tied into a knot.
[25] The hollow spaces in the bones of birds, however, are filled with air, which makes them lighter for flying..