eful
in movements of certain kinds. Of this class are the bones of the
spinal column, and ribs. Others are subservient to motion. Of this
class are the upper and lower extremities.

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119-128. _Give the physiology of the bones._ 119. How may the bones be
considered? 120. To what may the bones be compared? 121. Give the
different offices of the bones.

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122. The bones are subject to growth and decay; to removal of old,
useless matter, and the deposit of new particles, as in other tissues.
This has been tested by the following experiment. Some of the inferior
animals were fed with food that contained madder. In a few days, some
of the animals were killed, and their bones exhibited an unusually
reddish appearance. The remainder of the animals were, for a few
weeks, fed on food that contained no coloring principle. When they
were killed, their bones exhibited the usual color of such animals.
The coloring matter, which had been deposited, had been removed by the
action of the lymphatics.

123. The extremities of the bones that concur in forming a joint,
correspond by having their respective configurations reciprocal. They
are, in general, the one convex, and the other concave. In texture
they are porous, and consequently more elastic than if more compact.
These are covered with a cushion of cartilage. The elastic character
of these parts acts as so many springs, in diminishing the jar which
important organs of the system would otherwise receive.

124. The synovial membrane secretes a viscous fluid, which is called
_syn-o'vi-a_. This lubricating fluid of the joints enables the
surfaces of the bones and tendons to move smoothly upon each other,
thus diminishing the friction consequent on their action.

_Observations._ 1st. In this secretion is manifested the skill and
omnipotence of the Great Architect; for no machine of human invention
supplies to itself, by its own operations, the necessary lubricating
fluid. But, in the animal frame, it is supplied in proper quantities,
and applied in the proper place, and at the proper time.

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122. What is said of the change in bones? How was it proved that there
was a constant change in the osseous fabric? 123. What is said of the
extremities of the bones that form a joint? 124. What is synovia? Its
use? What is said of this lubricating fluid?

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2d. In some cases of in