A cross section of the spinal cord reveals white matter arranged around a butterfly-shaped area of gray matter. The white matter consists of myelinated fibres, or axons (axon), that form nerve tracts ascending to and descending from the brain. The white matter is grouped into discrete sectors called funiculi. The gray matter contains cell bodies, unmyelinated motor-neuron fibres, and interneurons connecting the two sides of the cord. Gray-matter cells form projections called horns. Fibres exiting the spinal cord from the dorsal and ventral horns join in paired tracts to form the spinal nerves (spinal nerve). Information travels up the ascending tracts of neurons and is sorted by the brain. Responses are induced by nerve impulses traveling down the descending tracts that stimulate motor neurons or that initiate glandular secretion. (See also nervous system, human.)