The first _cervical nerves_ pass out of the spinal column above the first cervical vertebra and the other cervical nerves below that and the succeeding vertebræ, while the other spinal nerves emerge each below the corresponding vertebra, as the first dorsal below the first dorsal vertebra, etc. After emerging they break up into a large anterior division and a small posterior division, the posterior branches supplying the spine and the dorsal muscles and skin, the anterior the rest of the trunk and the limbs. The _cervical plexus_ is formed by the anterior divisions of the first four cervical nerves, the _brachial plexus_ by the last four cervical and the first dorsal or thoracic nerves, the _lumbar plexus_ by the four upper lumbar, and the _sacral plexus_ by the last lumbar and the four upper sacral nerves. The only important branch of any of the four upper cervical nerves, which in general supply the neck and shoulders, is the _phrenic_, which is distributed to the pericardium, the pleuræ, and the under surface of the diaphragm. The _brachial plexus_, as its name implies, supplies the arms and has a number of important branches, as the circumflex to the shoulder, the musculo-cutaneous to the upper arm, the elbow-joint, and the outer surface of the forearm, the internal cutaneous to the inner side of the arm, the median to the pronators and flexors and the fingers on the radial side, and the ulnar to the elbow and wrist-joint. The musculo-spiral runs down the spiral groove to the external condyle of the humerus or upper arm bone, where it divides into the radial and the posterior interosseous, the former going to the thumb and two adjacent fingers and the latter to the wrist-joint and the muscles on the back of the forearm. Sometimes, in fracture of the humerus the callus thrown out pinches the musculo-spiral and causes pain. The dorsal or _thoracic nerves_ supply the back with their posterior divisions and their anterior divisions are the _intercostal nerves_. The _lumbar nerves_ supply the abdomen, pelvis, and thigh, the chief branches being the ilio-hypogastric to the abdomen and gluteal region, the ilio-inguinal to the inguinal region and scrotum, the external cutaneous and genito-crural to the thigh, and the obturator to the thigh and the hip and knee-joints. The anterior crural descends beneath Poupart’s ligament and divides into an anterior and a posterior division which supply the thigh muscles, its branches going to the pelvis. [Illustration: FIG. 30.--Diagrammatic view of the sympathetic cord of the right side, showing its connections with the principal cerebro-spinal nerves and the main preaortic plexuses. (Reduced from Quain’s anatomy.)]