The brain's white matter consists of a large mass of axons, the insulated nerve fibers that form the connections between neurons in the brain. The axon is the part of a nerve cell that carries the cell's electrical signals to other neurons, reports Mayfield Clinic.
The brain's nerve cells, or neurons, consist of three primary parts: the cell body, dendrites and the axon, according to the National Institute of Neurological Disorders and Stroke. The cell body houses the neuron's nucleus and is responsible for the overall functioning and survival of the neuron. The dendrites are small arms that branch out from the cell body and receive signals from other neurons. The axon is a long bundle of nerve fibers that carries electrical signals from the cell body to the end of the axon, where those signals can then be detected by the dendrites of other nearby neurons.
A fatty substance called myelin surrounds and insulates the axon, which serves to facilitate the transmission of electrical signals, explains MedlinePlus. Myelin also gives axons their white color, which is the origin of the term "white matter." The cell bodies of neurons make up the cortex, or gray matter, at the surface of the brain, while the axons form connections with other neurons by extending deeply through the inner parts of the brain. Because the axons are myelinated, they give those inner areas of the brain a distinct white color, giving rise to the label "white matter."