[Illustration:  Fig. 16.—­The Spinal Column.]

This column supports the head, encloses and protects the spinal cord, and forms the basis for the attachment of many muscles, especially those which maintain the body in an erect position.  Each vertebra has an opening through its center, and the separate bones so rest, one upon another, that these openings form a continuous canal from the head to the lower part of the spine.  The great nerve, known as the spinal cord, extends from the cranium through the entire length of this canal.  All along the spinal column, and between each two adjoining bones, are openings on each side, through which nerves pass out to be distributed to various parts of the body.

Between the vertebrae are pads or cushions of cartilage.  These act as “buffers,” and serve to give the spine strength and elasticity and to prevent friction of one bone on another.  Each vertebra consists of a body, the solid central portion, and a number of projections called processes.  Those which spring from the posterior of each arch are the spinous processes.  In the dorsal region they are plainly seen and felt in thin persons.

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 vertebrae 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 vertebrae 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 vertebrae 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 degrees or 70 degrees.  Now bend forwards, so as to let the dorsal and lumbar vertebrae come into play, and the head can be turned 30 degrees more.

Experiment 9. To show how the spinal vertebrae 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 vertebrae.  The most striking features of the