The Skull and Spinal Column

THE SKULL The skull is made up of 22 flat or irregular bones. Fourteen are facial, including those for cheeks, jaw, and upper bridge of the nose. Air spaces, or sinuses, in many of the facial bones serve to reduce the weight of the skull. Eight bones form the cranium, which protects the brain. There are additional bones in the head area: the hyoid, to which are attached the muscles that move the tongue; and the auditory ossicles .

In the middle ear-hammer, anvil and stirrup-which respond to sound waves hitting the eardrum with a lever action that transmits the waves to the inner ear.

THE SPINAL COLUMN The spinal column-a flexible stack of vertebrae-serves to support the head and trunk and also to protect the spinal cord, which extends down- ward from the brain. Each vertebra is shaped like a circle with the back side of the circle made up of a solid cylinder of bone. Running through the hollow part of the circle, the spinal cord shoots out branches of nerves that go to various parts of the body. As they stack toward the skull, the vertebrae gradually decrease in size. The vertebra just above the sacrum at the base is a heavy, large bone with large projections on each side and at the back, to which muscles and ligaments are attached. At the base of the skull, the top vertebra is a delicate bony ring with small protuberances. Between each vertebra and the next is a spinal disk-a circular cushion of connective tissue and cartilage about one-half-inch thick. Each disk has several layers of tough, fibrous rings and a softer nucleus in the center.