jacent vertebrae are disks of elastic
cartilage. Each disk is about one fourth of an inch thick and is grown
tight onto the face of the vertebra above and also onto the face of the
vertebra below. By means of these disks a very close connection is secured
between the vertebrae on the front side of the column.

_b._ On the back of the column, the downward projections from the neural
arch of each vertebra above fit into depressions found in the neural arch
of the vertebra below. This _interlocking_ of the vertebrae, which is most
marked in the lumbar region, strengthens greatly the back portion of the
column.

_c._ To further secure one bone upon the other, numerous ligaments pass
from vertebra to vertebra on all sides of the column.

2. _The Skull._--The skull is formed by the close union of twenty-two
irregular bones. These fall naturally into two subgroups--the cranium and
the face (Fig. 100). The _cranium_ consists of eight thin, curved bones
which inclose the space, called the _cranial cavity_, that holds the
brain. The _face group_, consisting of fourteen bones, provides cavities
and supports for the different organs of the face, and supplies a movable
part (the inferior maxillary) which, with the bones above (superior
maxillary), forms the machine for masticating the food.

[Fig. 100]

Fig. 100--*The skull (Huxley).* The illustration shows most of the bones of
the skull.

3. _The Thorax._--This group contains twenty-four bones of similar form,
called _ribs_, and a straight flat bone, called the _sternum_, or
breastbone (Fig. 101). The ribs connect with the spinal column behind, and
all but the two lowest ones connect with the sternum in front, and, by so
doing, inclose the thoracic cavity. As already stated (page 85), the bones
of the thorax form a mechanical device, or machine, for breathing. The
ribs are so arranged that the volume of the thorax is increased by
elevating them and diminished by depressing them, enabling the air to be
forced into and out of the lungs.

[Fig. 101]

Fig. 101--*Bone groups of trunk.*

4. _The Shoulder and Pelvic Girdles._--These groups form two bony
supports--one at the upper and the other at the lower portion of the
trunk--which serve for the attachment of the arms and legs (Fig. 101). The
_shoulder girdle_ is formed by four bones--two clavicles, or co