The thirty-one pairs of nerves of the somatic nervous system, which enter and leave the spinal cord between the spinal vertebrae. They are classified into five groups according to the part of the cord with which they are associated: (1) the eight cervical nerves are in the neck area, (2) the twelve thoracic nerves are in the chest area, (3) the five lumbar nerves are in the loin, (4) the five sacral nerves are near the base of the spinal column, and (5) the one coccygeal nerve is at the bottom end of the column.Just before entering the vertebral column, the spinal nerves divide into a dorsal root which is sensory in function and a ventral root which is usually motor. The sensory portions of the nerves come from tactile, pain and thermal receptors in the skin; receptors in the blood vessels; pressure and pain receptors in the muscles, tendons and joints; and from certain internal receptors in the body cavities and digestive tract. Each sensory nerve represents a relatively small body area called a dermatome. In lower animals these areas are sharply defined segments, but in man they are less regular and overlap considerably. The motor portions of the nerves control all the striated muscles of the arms, legs, and the rest of the body below the neck. The face, neck and head regions are not served by the spinal nerves, but by the cranial nerves.