**Core Concept**
The question is testing the student's knowledge of the anatomy of the structures within the spinal canal. Specifically, it is asking about the structures that are confined to one side of the spinal canal and do not cross the midline. This requires an understanding of the anatomy of the spinal cord and its associated structures.

**Why the Correct Answer is Right**
The correct answer is the spinal nerve root. Spinal nerve roots are the nerves that arise from the spinal cord and are responsible for transmitting sensory and motor signals between the central nervous system and the rest of the body. Unlike the spinal cord itself, which is a continuation of the brain and extends through the spinal canal, spinal nerve roots are confined to one side of the spinal canal and do not cross the midline. This is because the spinal nerve roots arise from specific nerve root levels and do not extend across the spinal canal.

**Why Each Wrong Option is Incorrect**
* **Option A:** The spinal cord itself does cross the midline, as it is a continuation of the brain and extends through the spinal canal.
* **Option B:** The spinal dura mater is a protective membrane that surrounds the spinal cord and nerve roots, but it does not contain structures that do not cross the midline.
* **Option C:** The spinal meninges are protective membranes that surround the spinal cord and nerve roots, but they do not contain structures that do not cross the midline.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the spinal nerve roots are responsible for transmitting sensory and motor signals between the central nervous system and the rest of the body. Damage to spinal nerve roots can result in a range of neurological symptoms, including pain, numbness, and weakness.

**Correct Answer:** C. The spinal nerve root.