minute cavities called _lacunæ_, while the fine lines are very minute
canals, _canaliculi_, which connect the lacunæ and the Haversian canals.
These Haversian canals are supplied with tiny blood-vessels, while the
lacunæ contain bone cells. Very fine branches from these cells pass into
the canaliculi. The Haversian canals run lengthwise of the bone; hence if
the bone be divided longitudinally these canals will be opened along their
length (Fig. 13).

Thus bones are not dry, lifeless substances, but are the very type of
activity and change. In life they are richly supplied with blood from the
nutrient artery and from the periosteum, by an endless network of
nourishing canals throughout their whole structure. Bone has, therefore,
like all other living structures, a _self-formative_ power, and draws from
the blood the materials for its own nutrition.

[Illustration: Fig. 13.

A, longitudinal section of bone, by which the Haversian canals are seen
branching and communicating with one another;
B, cross section of a very thin slice of bone, magnified about 300
diameters--little openings (Haversian canals) are seen, and around
them are ranged rings of bones with little black bodies (lacunæ), from
which branch out fine dark lines (canaliculi);
C, a bone cell, highly magnified, lying in lacuna.
]



The Bones of the Head.