shed.*--The bone cells, like all the other
cells of the body, are nourished by the lymph that escapes from the blood.
This passes through the canaliculi to the cells in the different parts of
the bone, as follows:

1. The cells in the surface layer of the bone receive lymph from the
capillaries in the periosteum.(79) It gets to them through the short
canaliculi that run out to the surface.

2. The cells within the interior of the bone receive their nourishment
from the small blood vessels in the Haversian canals. Lymph from these
vessels is conveyed to the cells through the canaliculi that connect with
the Haversian canals.

*Plan and Purpose of the Skeleton.*--The framework of the body is such as
to adapt it to a _movable_ structure. Obviously the different parts of the
body cannot be secured to a foundation, as are those of a stationary
building, but must be arranged after a plan that is conducive to motion. A
moving structure, as a wagon or a bicycle, has within it some strong
central part to which the remainder is joined. The same is true of the
skeleton. That part to which the others are attached is a long, bony axis,
known as the _spinal column_. Certain parts, as the ribs and the skull,
are attached directly to the spinal column, while others are attached
indirectly to it. The arrangement of all the parts is such that the spinal
column is made the central, cohering portion of the skeleton and also of
the whole body.

Besides the general arrangement of the parts of the skeleton, there is
such a grouping of the bones in each of its main divisions as will enable
them to serve definite purposes. In most places they form mechanical
devices for supplying special movements, and in certain places they
provide for the support or protection of important organs. In most cases
there is a definite combination of different bones, forming what is called
the bone group.

[Fig. 97]

Fig. 97--The human skeleton.

*Bone Groups.*--On account of the close relation between the bones of the
same group, they cannot profitably be studied as individual bones, but
each must be considered as a part of the group to which it belongs. By
first making out the relation of a given bone to its group, its value to
the whole body can be determined. The most important of the groups of
bones are as follows:

1. _The Spinal Column._--This group consists of twenty-four similarly
shaped