38.  How the Head and Spine are Joined together.  The head rests upon the spinal column in a manner worthy of special notice.  This consists in the peculiar structure of the first two cervical vertebrae, known as the axis and atlas.  The atlas is named after the fabled giant who supported the earth on his shoulders.  This vertebra consists of a ring of bone, having two cup-like sockets into which fit two bony projections arising on either side of the great opening (foramen magnum) in the occipital bone.  The hinge joint thus formed allows the head to nod forward, while ligaments prevent it from moving too far.

On the upper surface of the axis, the second vertebra, is a peg or process, called the odontoid process from its resemblance to a tooth.  This peg forms a pivot upon which the head with the atlas turns.  It is held in its place against the front inner surface of the atlas by a band of strong ligaments, which also prevents it from pressing on the delicate spinal cord.  Thus, when 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.