this bulky material diminishes the shock of a sudden blow, and also acts
as a beam to brace the exterior wall. The recumbent position of the
alligator protects him from falls, and therefore his bones contain very
little spongy substance.

In the body, bones are not the dry, dead, blanched things they commonly
seem to be, but are moist, living, pinkish structures, covered with a
tough membrane, called the per-i-os'-te-um [Footnote: The relations of the
periosteum to the bone are very interesting. Instances are on record where
the bone has been removed, leaving the periosteum, from which the entire
bone was afterward renewed.] (_peri_, around, and _osteon_, a
bone), while the hollow is filled with marrow, rich in fat, and full of
blood vessels. If we examine a thin slice with the microscope, we shall
see black spots with lines running in all directions, and looking very
like minute insects. These are really little cavities, called la-cu'-ne
[Footnote: When the bone is dry, the lacune are filled with air, which
refracts the light, so that none of it reaches the eye, and hence the
cavities appear black.] from which radiate tiny tubes. The lacune are
arranged in circles around larger tubes, termed from their discoverer,
_Haversian canals_, which serve as passages for the blood vessels
that nourish the bone.

GROWTH OF THE BONES.--By means of this system of canals, the blood
circulates as freely through the bones as through any part of the body,
The whole structure is constantly but slowly changing, [Footnote: Bone is
sometimes produced with surprising rapidity. The great Irish Elk is
calculated by Prof. Owen to have cast off and renewed, annually in its
antlers eighty pounds of bone.] old material being taken out and new put
in. A curious illustration is seen in the fact that if madder be mixed
with the food of pigs, it will tinge their bones red.