Posterior Primary Division.

The posterior primary division of the spinal nerves nearly all divide into internal and external branches, which supply the muscles and skin of the back. The first cervical, two lowest sacral, and coccygeal are the nerves which do not divide.

Anterior Primary Division.

The various plexuses of nerves are formed by nerves from the anterior primary division. A plexus means an interlacing of nerves or bloodvessels ; in this case it means nerves. The nerves from this division supply the limbs and the front and side of the neck, thorax, and abdomen.