spaces, and the rows of cells are seen
resting in the cavities so formed, the nuclei being
more separated than above. _3_'. Portion of the
same more highly magnified.]

[Illustration: Fig. 8.
Thigh-bone,
sawn open
lengthwise.]

[Illustration: Fig. 9.
Lower end of the thigh-bone
sawn across, showing its central
cavity.]

The bony plates form the boundaries of the _Haversian_, or nutritive
canals of the bones. In the _second stage of ossification_, the
cartilage corpuscles are converted into bone. Becoming flattened against
the osseous lamellæ already formed, they crowd upon one another so as to
entirely obliterate the lines that distinguish them; and, simultaneously
with these changes, a calcareous deposit takes place upon their
interior. Bones grow by additions to their ends and surfaces. In the
child, their extremities are separated from the body of the bone by
layer of cartilage, and the cancellated, or cellular structure, which
remains for a time in the interior, represents the early condition of
the ossifying substances.

The bones contain more earthy matter in their composition than any other
part of the human body, being firm, hard, and of a lime color. They
compose the skeleton or frame work, and, when united by natural