The bones of the spinal column are arranged in three slight and graceful curves. These curves not only give beauty and strength to the bony framework of the body, but also assist in the formation of cavities for important internal organs. This arrangement of elastic pads between the vertebræ supplies the spine with so many elastic springs, which serve to break the effect of shock to the brain and the spinal cord from any sudden jar or injury. The spinal column rests on a strong three-sided bone called the sacrum, or sacred-bone, which is wedged in between the hip bones and forms the keystone of the pelvis. Joined to the lower end of the sacrum is the coccyx, or cuckoo-bone, a tapering series of little bones. Experiment 7. Run the tips of the fingers briskly down the backbone, and the spines of the vertebræ will be tipped with red so that they can be readily counted. Have the model lean forward with the arms folded across the chest; this will make the spines of the vertebræ more prominent. Experiment 8. _To illustrate the movement of torsion in the spine, or its rotation round its own axis_. Sit upright, with the back and shoulders well applied against the back of a chair. Note that the head and neck can be turned as far as 60° or 70°. Now bend forwards, so as to let the dorsal and lumbar vertebræ come into play, and the head can be turned 30° more. Experiment 9. _To show how the spinal vertebræ make a firm but flexible column._ Take 24 hard rubber overcoat buttons, or the same number of two-cent pieces, and pile them on top of each other. A thin layer of soft putty may be put between the coins to represent the pads of cartilage between the vertebræ. The most striking features of the spinal column may be illustrated by this simple apparatus. 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 vertebræ, 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.