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Atlas - Spinal membranes and nerve roots

The spinal cord is surrounded by the meninges,, which also surround the brain and brain stem, and cerebral spinal fluid. The three layers of meninges that surround the spinal cord are the spinal dura mater, the spinal arachnoid mater and the spinal pia mater. The spinal pia mater, which is the deepest investing layer of the spinal meninges, is vascularised and contains numerous small blood vessels that penetrate the spinal cord known as the vascular plexus of the pia mater. The anterior spinal artery courses along the anterior aspect of the spinal cord from the level of the foramen magnum all the way down to the filum terminale and gives off several branches along its course.

The spinal cord is comprised of white matter and gray matter, and the gray matter which has a anterior horn, a posterior horn and a lateral horn, is arranged around the central canal of the spinal cord. Rootlets of the anterior root emerge from the anterior horn of the spinal cord and merge to form the anterior root of spinal nerve.
In the same fashion, rootlets of the posterior root emerge from the posterior horn of the spinal cord and merge to form the posterior root of spinal nerve. The posterior root of spinal nerve carries afferent sensory fibers and contains a spinal ganglion.
The anterior and posterior roots of spinal nerves unite to form spinal nerves, of which there are 31 pairs and they carry mixed fibers. Shortly after the spinal nerve emerges from the intervertebral foramen branches into an anterior ramus, a posterior ramus and a rami communicans. Two rami communicantes, namely the gray ramus communicans and the white ramus communicans, connect the spinal nerve and the sympathetic trunk.

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