age, the bones grow harder, by
the gradual deposition of the phosphate of lime, which is supplied by
the food, and carried to the bones by the blood. In old age, the hardest
material preponderates; making the bones more brittle than in earlier
life.

As we shall soon have occasion to refer, particularly, to the spinal, or
vertebral column, and the derangement to which it is liable, we give, on
page 72, representations of the different classes of vertebrae; viz. the
_cervical_, (from the Latin, _cervix_, the neck,) the _dorsal_, (from
_dorsum_, the back,) and _lumbar_, (from _lumbus_, the loins.)

[Illustration: Fig. 2.]

Fig. 2, represents one of the _cervical vertebrae_. Seven of these,
placed one above another, constitute that part of the spine which is in
the neck.

[Illustration: Fig. 3.]

Fig. 3, is one of the _dorsal vertebrae_, twelve of which, form the
central part of the spine.

[Illustration: Fig. 4.]

Fig. 4, represents one of the _lumbar vertebrae_, (five in number,)
which are immediately above the sacrum. These vertebrae are so fastened,
that the spine can bend, in any direction; and the muscles of the trunk
are used in holding it erect, or in varying its movements.

By the drawings here presented, it will be seen, that the vertebrae of
the neck, back, and loins, differ somewhat in size and shape, although
they all possess the same constituent parts; thus, A, in each,
represents the body of the vertebrae; B, the articulating processes, by
which each is joined to its fellow, above and below it; C, the spinous
process, or that part of the vertebrae, which forms the ridge to be felt,
on pressure, the whole length of the centre of the back. The back bone
receives its name, _spine_, or _spinal column_, from these spinous
processes.

It is the universal law of the human frame, that _exercise_ is
indispensable to the health of the several parts. Thus, if a
blood-vessel be tied up, so as not to be used, it shrinks, and becomes a
useless string; if a muscle be condemned to inaction, it shrinks in
size, and diminishes in power; and thus it is also with the bones.
Inactivity produces softness, debility, and unfitness for the functions
they are designed to perform. This is one of the causes of the curvature
of the spine, that common and pernicious defect in the females of
America. From inactivity, the bones of the spine become soft and
yielding; and then, if the person is often placed, for a length of time