3. _In the midbrain, bulb, and spinal cord_ fibers are found: first, that connect these parts with the cerebrum(101) and cerebellum above; second, that pass into and become a part of the nerves of the body; and third, that connect the opposite sides of these parts together. *The Peripheral Division.*—The peripheral division of the nervous system includes all the nervous structures found outside of the brain and spinal cord. These consist of the cranial, spinal, and sympathetic nerves, and of various small ganglia, all of which are closely connected with the central system. *Spinal Nerves and Dorsal-root Ganglia.*—The spinal nerves comprise a group of thirty-one pairs, which connect the spinal cord with different parts of the trunk, with the upper, and with the lower extremities. Each nerve joins the cord by two roots, these being named from their positions the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The two roots blend together within the spinal cavity to form a single nerve trunk, which passes out between the vertebræ. On the dorsal root of each spinal nerve is a small ganglion which is named, from its position, the _dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.) *Double Nature of Spinal Nerves.*—Charles Bell, in 1811, made the remarkable discovery that each spinal nerve is double in function. He found the portion connecting with the cord by the dorsal root to be concerned in the _production of feeling_ and the portion connecting by the ventral root to be concerned in the _production of motion_. In keeping with these functions, the two divisions of the nerve are made up of different kinds of fibers, as follows: 1. The dorsal-root divisions, of the fibers of di-axonic neurons, the cell-bodies of which form the dorsal-root ganglia (Fig. 135). 2. The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord. The first convey impulses to the cord and are called _afferent_ neurons;(102) the second convey impulses from the cord and are known as _efferent_ neurons. Thus, by forming a part of the nerve pathways between the skin and the brain, the dorsal divisions of these nerves aid in the production of feeling; and by completing pathways to the muscles, the ventral divisions aid in the production of motion (Figs. 129, 135, and 141). [Fig. 135] Fig. 135—*Connection of spinal nerves with the cord.* On the right is shown a nerve pathway from the skin to the muscle. A division of this pathway reaches the brain.