Okay, let's tackle this question about the ligamentum denticulatum. The question is asking for the total number on both sides of the spinal cord. First, I need to recall what the ligamentum denticulatum is. From my anatomy classes, I remember that these are part of the denticulate ligaments, which are connected to the pia mater and help anchor the spinal cord within the spinal canal.

Wait, the denticulate ligaments are paired structures, right? Each has a series of tooth-like processes (the denticles) that attach to the arachnoid and dura. How many of these are there on each side? I think there are 21 on each side, extending from the C1 to the S2 levels. So if there are 21 on each side, the total would be 42 when combined for both sides.

Let me check the options. The correct answer is D, which is 21 on each side, totaling 42. The other options might be common mistakes. For example, someone might think there are 15 or 25 if they miscount or confuse with another structure. Another possible distractor could be 12, thinking of the number of thoracic vertebrae, but that's not relevant here.

The key point here is knowing the exact number of denticulate ligaments on each side and remembering that they are present from C1 to S2, which gives 21 segments. The clinical pearl is that these ligaments help prevent the spinal cord from shifting within the subarachnoid space, which is important in trauma or surgery to avoid damage. So the correct answer should be 21 on each side, making the total 42, hence option D.

**Core Concept**
The denticulate ligaments (ligamentum denticulatum) are paired fibrous bands extending from the pia mater to the arachnoid and dura, anchoring the spinal cord. They are present **unilaterally at each spinal segment** from C1 to S2, totaling 21 on each side in adults.

**Why the Correct Answer is Right**
Each denticulate ligament is a triangular, tooth-like structure that prevents lateral displacement of the spinal cord. In adults, there are **21 ligaments per side** (C1–C8, T1–T12, L1–L5, S1–S2), corresponding to the 33 spinal segments minus the fused sacrum/coccyx. The total bilateral count is **42**, but the question specifically asks for the number *on both sides combined*. If the correct answer is 21 (per side), the question may be ambiguously phrased or the options may reflect per-side counts. Clarification is needed, but standard anatomical references state 21 per side.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect. Likely represents a lower count, such as the number of cervical vertebrae (7), ignoring thoracolumbar segments.
**Option B:** Incorrect. May reflect a miscalculation (e.g., 15) or confusion with other spinal structures like intervertebral discs.
**Option C:** Incorrect. Could stem from miscounting thoracic segments (12)

✓ Correct Answer: D. 40-42