Saccular aneurysm:  A balloon-like outpouching of a vessel. More about aneurysms.
Sciatic nerve:  The largest nerve in the body. It extends from the sacral plexus, emerges from the pelvis, and travels deep within the buttocks. It then descends down the back of the thigh to the back of the knee, at which point it divides into the common peroneal and tibial nerves. The sciatic nerve supplies sensation to the back of the thigh, outer side of the leg, and essentially the whole foot.
Seizure:  Upper motor cortex
Shunt:  A tube that diverts body fluid from one body cavity or vessel to another.
Spasmodic torticollis:  Also known as cervical dystonia, is a form of dystonia characterized by intermittent spasms of the neck muscles resulting in involuntary rotation and tilting of the head. These movements are frequently painful. More about dystonia.
Spina bifida:  A congenital defect of the spine marked by the absence of a portion of the spine.
Spinal fusion:  Operative method of strengthening and limiting motion of the spinal column. Can be performed with a variety of metal instruments and bone grafts or bone grafts alone.
Spinal instability:  Damage to a disc or tearing of the ligament holding the spine, which causes the spine to be unable to carry out its supporting function.
Spinal tap:  See lumbar puncture
Spinous process:  A bony prominence projecting backward from a vertebra that can be felt under the skin on one's back.
Spondylolisthesis:  Forward displacement or slippage (subluxation) of one vertebra over another. More about spondylolisthesis.
Spondylosis:  Degenerative bone changes in the spine usually most marked at the vertebral joints with bony spur formation. More about ankylosing spondylosis.
Stenosis:  Narrowing of the openings of the foramen and/or the spinal canal; narrowing of a blood vessel. More about spinal stenosis.
Stent:  An endovascular prosthetic device, usually an open mesh cylinder, placed within a blood vessel to provide the support to keep that vessel open.
Stereotactic surgery:  Surgical technique whereby the exact target (i.e., tumor, lesion, AVM) is calculated three-dimensionally utilizing CT or MRI and computer. From stereo (three-dimensional) and tactic (touch).
Subarachnoid:  Located under the arachnoid membrane and above the pia mater.
Subarachnoid hemorrhage:  Blood in, or bleeding into, the space under the arachnoid membrane, most commonly from trauma or from rupture of an aneurysm. More about subarachnoid hemorrhage.
Subdural:  Located below the dura mater and above the arachnoid meninges.
Subdural hematoma:  A collection of blood (clot) trapped under the dura matter, the outermost membrane surrounding the brain and spinal cord.
Suprascapular nerve:  The nerve that originates from the superior trunk of the brachial plexus. It supplies the shoulder joint and deep shoulder structures.

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