Fig. 134—Semi-diagrammatic representation of a section through the right cerebral hemisphere, showing fiber tracts. A. Association fibers. C. Commissural fibers. P. Projection fibers. The cell-bodies with which the fiber bundles connect are in the surface layer or cortex.
3. In the midbrain, bulb, and spinal cord fibers are found: first, that connect these parts with the cerebrum[101] and cerebellum above; second,[pg 295] 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[pg 296] 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).
