What is the fatty tissue that covers axons and speeds up impulses as they travel from cell to cell?

The fatty tissue is called the Myelin sheath. The myelin sheath is a significantly extended and altered plasma membrane wrapped about the nerve axon in a spiral form. The myelin membranes somes from and are a part of the Schwann cells in the peripheral nervous system and the oligodendroglial cells in the central nervous system. Each myelin-spawning cell delivers myelin for only one segment of any given axon. The intermittent interruptions where short portions of the axon are left uncovered by myelin are the nodes of Ranvier, and they are critical to the functioning of myelin.