If the bone be burned in the fire, thus consuming the animal matter, the
shape will still be the same, but it will have lost its tenacity, and the
beautiful, pure-white residue [Footnote: From bones thus calcined, the
phosphorus of the chemist is made. See Steele's "Popular Chemistry," page
114. If the animal matter be not consumed, but only charred, the bone will
be black and brittle. In this way, the "boneblack" of commerce is
manufactured.] may be crumbled into powder with the fingers.

FIG. 2.

[Illustration: _The Thigh Bone, or Femur, sawed lengthwise._]

We thus see that a bone receives hardness and rigidity from its mineral,
and tenacity and elasticity from its animal matter.

The entire bone is at first composed of cartilage, which gradually
_ossifies_ or turns to bone. [Footnote: The ossification of the bones
on the sides and upper part of the skull, for example, begins by a rounded
spot in the middle of each one. From this spot the ossification extends
outward in every direction, thus gradually approaching the edges of the
bone. When two adjacent bones meet, there will be a line where their edges
are in contact with each other, but have not yet united; but when more
than two bones meet in this way, there will be an empty space between them
at their point of junction. Thus, if you lay down three coins upon the
table with their edges touching one another, there will be a three-sided
space in the middle between them; if you lay down four coins in the same
manner, the space between them will be four-sided. Now at the back part of
the head there is a spot where three bones come together in this way,
leaving a small, three-sided opening between them: this is called the