the mineral matter
is dissolved out, and as a result it loses its properties of hardness and
stiffness. (See Practical Work.) This is because the mineral matter
supplies these properties, being composed of substances which are hard and
closely resemble certain kinds of rock. The chief materials forming the
mineral matter are calcium phosphate and calcium carbonate.

On the other hand, burning a bone destroys the animal matter. When this is
done the bone loses its toughness, and becomes quite brittle. The property
of toughness is, therefore, supplied by the animal matter. This consists
mainly of a substance called _ossein_, which may be dissolved out of the
bones by boiling them. Separated from the bones it is known as _gelatine_.
The blood vessels and nerves in the bones, and the protoplasm of the bone
cells, are also counted in with the animal matter.

[Fig. 93]

Fig. 93--*Section of a long bone* (_tibia_), showing the gross structure.

If a dry bone from a full-grown, but not old, animal be weighed before and
after being burned, it is found to lose about one third of its weight.
From this we may conclude that about one third of the bone by weight is
animal matter and two thirds is mineral matter. This proportion, however,
varies with age, the mineral matter increasing with advance of years.

*Gross Structure of Bones.*--The gross structure of the bones is best
learned by studying both dry and fresh specimens. (See Practical Work.)
The ends of the bones are capped by a layer of smooth, elastic cartilage,
while all the remaining surface is covered by a rather dense sheath of
connective tissue, called the _periosteum_. Usually the central part of
the long bones is hollow, being filled with a fatty substance known as the
_yellow marrow_. Around the marrow cavity the bone is very dense and
compact, but most of the material forming the ends is porous and spongy.
These materials are usually referred to as the _compact substance_ and the
_cancellous_, or _spongy, substance_ of the bones (Fig. 93).

The arrangement of the compact and spongy substance varies with the
different bones. In the short bones (wrist and ankle bones, vertebrae,
etc.) and also in the flat bones (skull bones, ribs, shoulder blades,
etc.) there is no cavity for the yellow marrow, all of the interior space
being filled with the spongy substance. The _red marrow_, relations of
which to the red corpuscles of the bloo