we turn the head to the right or left, the skull
and the atlas move together, both rotating on the odontoid process of the
axis.

39. The Ribs and Sternum. The barrel-shaped framework of the chest is
in part composed of long, slender, curved bones called ribs. There
are twelve ribs on each side, which enclose and strengthen the chest; they
somewhat resemble the hoops of a barrel. They are connected in pairs with
the dorsal vertebrae behind.

The first seven pairs, counting from the neck, are called the _true_ ribs,
and are joined by their own special cartilages directly to the breastbone.
The five lower pairs, called the _false_ ribs, are not directly joined to
the breastbone, but are connected, with the exception of the last two,
with each other and with the last true ribs by cartilages. These elastic
cartilages enable the chest to bear great blows with impunity. A blow on
the sternum is distributed over fourteen elastic arches. The lowest two
pairs of false ribs, are not joined even by cartilages, but are quite free
in front, and for this reason are called _floating_ ribs.

The ribs are not horizontal, but slope downwards from the backbone, so
that when raised or depressed by the strong intercostal muscles, the size
of the chest is alternately increased or diminished. This movement of the
ribs is of the utmost importance in breathing (Fig. 91).

The sternum, or breastbone, is a long, flat, narrow bone forming the
middle front wall of the chest. It is connected with the ribs and with the
collar bones. In shape it somewhat resembles an ancient dagger.

40. The Hip Bones. Four immovable bones are joined together so as to
form at the lower extremity of the trunk a basin-like cavity called the
pelvis. These four bones are the sacrum and the coccyx,
which have been described, and the two hip bones.

[Illustration: Fig. 17.--Thorax. (Anterior view.)]

The hip bones are large, irregularly shaped bones, very firm and
strong, and are sometimes called the haunch bones or _ossa innominata_
(nameless bones). They are united to the sacrum behind and joined to each
other in front. On the outer side of each hip bone is a deep cup, or
socket, called the _acetabulum_, resembling an ancient vinegar cup, into
which fits the rounded head of the thigh bone. The bones of the pelvis are
supported like a bridge on the legs as pillars, and they in turn contain
the internal organs in the lower part of the trunk.

41. The Hyoid Bone.