The peripheral system is somewhat complex and numerous intercommunications are established by which nerve impulses originating in the central axis and leaving by one part of the peripheral system may exercise a controlling influence over another part. Plexuses, or intertwinings of nerve axons, are so numerous and complicated that it is difficult to follow each set of nerve stimuli from their origin to their final destination and effect without considerable study. Direct Distribution of Spinal Axons The spinal nerve axons, taken as a whole, establish paths between the motor gray of the ventral horn of the spinal cord and all voluntary muscles of the body below the head except the trapezius and sternomastoid, partially innervated by the eleventh cranial, and between the sensor cells of the dorsal spinal gray and gracile and cuneate nuclei of the medulla on the one hand and the sensor end organs in skin and mucuous membrane, muscles, tendons, and joints on the other. The ventral cornu receives impulses from the cortico-spinal axons of the direct pyramidal, crossed pyramidal, rubrospinal, and other smaller tracts which bring the spinal gray under the direct voluntary domination of the volitional centers in the brain or of the indirectly voluntary pathway through the cerebellum. The spinal nerves are the direct media for motion of the body or its parts in relation to its environment. The sensor gray of the cord is similarly in communication with the conscious sensation area in the cerebrum and with the cerebellum by way of the dorsal tracts of the cord, the lemnisci, and the cerebellar peduncles. In the main these nerves of motion and sensation are arranged as follows: The Cervical plexus is composed of the intertwining of axons from the anterior primary divisions of the four upper Cervical nerves. Its branches pass to and innervate many voluntary muscles of the neck and side and back of head, and supply sensor fibres to the adjacent cutaneous areas. Branches also communicate with the last three cranial nerves and one long branch, the Phrenic, or Internal Respiratory Nerve of Bell, passes through the neck and thorax to the diaphragm, as its motor nerve. The Brachial plexus is made up of the anterior primary divisions of the four lower Cervical nerves and the greater part of the first Thoracic. It is distributed chiefly to the voluntary muscles and integument of the shoulder and arm, forearm, and hand, but sends branches to some muscles of the neck and upper back as well. It, like the Cervical plexus, receives branches from, but gives none to, the Cervical sympathetic. The Thoracic nerves are not arranged in plexiform fashion like those above but pass separately, for the most part, to their destinations. They are distributed to the walls of the thorax and abdomen following the curve of the ribs in direction. The last Thoracic sends one division downward as far as the outer aspect of the ilium.