d have already been noted (page
27), occupies the minute spaces in the spongy substance.

[Fig. 94]

Fig. 94--*Cross section of bone showing minute structure.* Magnified. 1.
Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which
pass the canaliculi. 4. A lacuna. Observe arrangement of lacunae at surface
and in deeper portion.

*Minute Structure of Bone.*--A microscopic examination of a thin slice of
bone taken from the compact substance shows this to be porous as well as
the spongy substance. Two kinds of small channels are found running
through it in different directions, known as the Haversian canals and the
canaliculi (Fig. 94). These serve the general purpose of distributing
nourishment through the bone. The _Haversian canals_ are larger than the
canaliculi and contain small nerves and blood vessels, chiefly capillaries
(Fig. 95). They extend lengthwise through the bone. The _canaliculi_ are
channels for conveying lymph. They pass out from the Haversian canals at
right angles, going to all portions of the compact substance except a thin
layer at the surface. In the surface layer of the bone the canaliculi are
in communication with the periosteum.

[Fig. 95]

Fig. 95--*Section showing Haversian canal and contents*, highly magnified
(after Schaefer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve
fibers. 4. Lymph vessel.

*The Bone Cells.*--Surrounding the Haversian canals are thin layers of bone
substance called the _laminae_, and within these are great numbers of
irregular bodies, known as the _lacunae_. The walls of the lacunae are hard
and dense, but within each is an open space. In this lies a flattened
body, having a nucleus, which is recognized as the _bone cell_, or the
bone corpuscle (Fig. 96). It appears to be the work of the bone cells to
deposit mineral matter in the walls surrounding them and in this way to
supply the properties of hardness and stiffness to the bones. The
canaliculi connect with the lacunae in all parts of the bone, causing them
to appear under the microscope like so many burs fastened together by
their projecting spines (Fig. 94).

[Fig. 96]

Fig. 96--*Bone cell* removed from the lacuna and very highly magnified.
(From Quain's _Anatomy_.)

*How the Bone Cells are Nouri