The brain and spinal cord form the central nervous system and are protected by the bones of the skull and the vertebral column. Inside these hard outer coverings, further protection is provided by three membranes called the meninges and a clear liquid known as cerebrospinal fluid. The cranial nerves emerge directly from the brain, and the spinal nerves from the spinal cord. Together, these form the peripheral nerves that branch to every part of the body.
Three membranes (the dura mater, arachnoid, and pia mater) surround and protect the brain and spinal cord, which are also cushioned by protective cerebrospinal fluid.
Sciatic nerve fascicle
This cross section shows a fascicle, which is a bundle of nerve fibres, in the sciatic nerve. Individual nerve fibres may be very long, extending to 1 m (3 ft).
A nerve consists of hundreds of nerve fibres grouped in bundles called fascicles. The larger fibres are insulated by sheaths made of a fatty substance called myelin.
The delicate spinal cord is protected by the bony vertebral column. Nerves emerge from the cord between the vertebrae.