resh or living bone is covered with a delicate, tough, fibrous
membrane, called the periosteum. It adheres very closely to the bone,
and covers every part except at the joints and where it is protected with
cartilage. The periosteum is richly supplied with blood-vessels, and plays
a chief part in the growth, formation, and repair of bone. If a portion of
the periosteum be detached by injury or disease, there is risk that a
layer of the subjacent bone will lose its vitality and be cast off.[5]

30. Microscopic Structure of Bone. If a very thin slice of bone be
cut from the compact tissue and examined under a microscope, numerous
minute openings are seen. Around these are arranged rings of bone, with
little black bodies in them, from which radiate fine, dark lines. These
openings are sections of canals called _Haversian canals_, after Havers,
an English physician, who first discovered them. The black bodies are
minute cavities called _lacunae_, while the fine lines are very minute
canals, _canaliculi_, which connect the lacunae and the Haversian canals.
These Haversian canals are supplied with tiny blood-vessels, while the
lacunae 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 (lacunae), from
which branch out fine dark lines (canaliculi);
C, a bone cell, highly magnified, lying in lacuna.
]

The Bones of the Head.

31. The Head, or Skull. The bones of the skeleton, the bony framework
of our bodies, may be divided into those of the head, the trunk,
and the limbs.

The bones