belonging to the back. All these processes are more compact than the
body of the vertebra, and, when naturally connected, are so arranged as
to form a tube which contains the _medulla spinalis_, or spinal cord.
Between the vertebræ is a highly-elastic, cartilaginous and cushion-like
substance, which freely admits of motion, and allows the spine to bend
as occasion requires. The natural curvatures of the spinal column
diminish the shock produced by falling, running or leaping, which would
otherwise be more directly transmitted to the brain. The ribs at the
sides, the sternum in front, and the twelve dorsal bones of the spinal
column behind, bound the thoracic cavity, which contains the lungs,
heart, and large blood-vessels.

[Illustration: Fig. 15.
A representation of the pelvic bones. _e_. The
lumbo-sacral joint. 2. The sacrum. _3_. Coccyx. _1,1_.
The innominata. _4,4_. Acetabula.]

The _Pelvis_ is an open bony structure, consisting of the Os Innominata,
one on either side, and the Sacrum and Coccyx behind. The _Sacrum_,
during childhood, consists of five bones, which in later years unite to
form one bone. It is light and spongy in texture, and the upper surface
articulates with the lowest vertebra, while it is united at its inferior
margin to the coccyx. The _Coccyx_ is the terminal bone of the spinal
column. In infancy it is cartilaginous and composed of several pieces,
but in the adult these unite and form one bone. The _Innominata_, or
nameless bones, during youth, consist of three separate pieces on each
side; but as age advances they coalesce and form one bone. A deep
socket, called the _acetabulum_, is found near their junction, which
serves for the reception of the head of the thigh-bone.