**Core Concept**
Nysten's law describes the relationship between the distance of the point of fixation and the distance of the point of accommodation in the human eye. It is a fundamental principle in ophthalmology and optometry that helps in understanding how the eye accommodates to focus on objects at different distances.

**Why the Correct Answer is Right**
Nysten's law states that the distance of the point of fixation (F) plus the distance of the point of accommodation (A) is equal to the sum of the near point (F-N) and the far point (F-F). In mathematical terms, F + A = F-N + F-F. This law is essential in understanding how the eye focuses on objects at varying distances and how it adjusts to accommodate for different visual demands.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not accurately describe Nysten's law. The correct relationship should be F + A = F-N + F-F.
* **Option B:** This option is incorrect as it is a description of another ophthalmological concept, not Nysten's law. Nysten's law specifically deals with the relationship between distance of fixation and accommodation.
* **Option C:** This option is incorrect as it is a description of the near point and far point of vision, which are related but not directly equivalent to Nysten's law.

**Clinical Pearl / High-Yield Fact**
Nysten's law is used to calculate the near point and far point of vision in patients with refractive errors or other ocular conditions. It is an essential tool in ophthalmic practice for assessing visual acuity and prescribing corrective lenses.

**Correct Answer: D. Nysten's law states that the distance of the point of fixation (F) plus the distance of the point of accommodation (A) is equal to the sum of the near point (F-N) and the far point (F-F).**

✓ Correct Answer: D. Cadaveric rigidity affect successively the masticatory muscles, those of the face and the neck, those of the trunk and arms and finally those of the lower limbs