system to the offices they are required to perform? 66. What is
a natural skeleton? What an artificial? 67. What part of the bones are
called processes? 68-73. _Give the structure of the bones._ 68. Of
what are the bones composed? What are the different uses of the
component parts of the bones?

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_Experiments._ 1st. To show the earthy without the animal matter, burn
a bone in a clear fire for about fifteen minutes, and it becomes white
and brittle, because the gelatin, or animal matter of the bone, has
been destroyed.

2d. To show the animal without the earthy matter of the bones, immerse
a slender bone for a few days in a weak acid, (one part muriatic acid
and six parts water,) and it can then be bent in any direction. In
this experiment, the acid has removed the earthy matter, (carbonate
and phosphate of lime,) yet the form of the bone is unchanged.

69. The bones are formed from the blood, and are subjected to several
changes before they are perfected. At their early formative stage,
they are cartilaginous. The vessels of the cartilage, at this period,
convey only the _lymph_, or white portion of the blood; subsequently,
they convey red blood. At this time, true ossification (the deposition
of phosphate and carbonate of lime) commences at certain points, which
are called _the points of ossification_.

70. Most of the bones are formed of several pieces, or centres of
ossification. This is seen in the long bones which have their
extremities separated from the body by a thin partition of cartilage.
It is some time before these separate pieces are united to form one
bone.

71. When the process of ossification is completed, there is still a
constant change in the bones. They increase in bulk, and become less
vascular, until middle age. In advanced life, the elevations upon
their surface and near the extremities become more prominent,
particularly in individuals accustomed to labor. As a person advances
in years, the vitality diminishes, and in extreme old age, the earthy
substance predominates; consequently, the bones are extremely
brittle.

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How can the earthy matter of the bones be shown? The animal? 69.
What is the appearance of the bones in their early formative stage?
When does true ossification commence? 70. How are most of the bones
formed? 71. What is said of the various changes of the bones after
ossification?

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72. T