The insulating substance on axons is:

1. It is lipid rich and makes white matter white. 2. It works like the plastic on a wire. 3. Axons are insulated by a lipid-rich substance called myelin. 4. So the answer is myelin. _Source: OpenStax 'Anatomy and Physiology 2e', Ch 12 'The Nervous System and Nervous Tissue'_