**Core Concept**
Harderian gland, also known as Harder's gland or Harder's organ, is a small gland located in the orbit, specifically in the retrobulbar space. It is a specialized sebaceous gland that secretes a waxy substance called harderian secretion or harderian oil, which is thought to have antimicrobial properties and possibly play a role in the regulation of the eye's circadian rhythm.

**Why the Correct Answer is Right**
The harderian gland is a unique structure that is found in many vertebrates, including mammals, birds, and reptiles. It is believed to have evolved as a defense mechanism against pathogens and environmental stressors. The harderian secretion is composed of lipids, proteins, and other compounds that are thought to have antimicrobial and antifungal properties. In some species, the harderian gland is also involved in the regulation of the eye's circadian rhythm, with changes in secretion levels influencing the pupillary light reflex.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is not a known function or characteristic of the harderian gland.
* **Option B:** This is not a correct description of the harderian gland's location or function.
* **Option C:** This is not a correct description of the harderian gland's role or properties.

**Clinical Pearl / High-Yield Fact**
The harderian gland is often overlooked in clinical practice, but it may play a role in certain eye diseases, such as conjunctivitis or uveitis, where its antimicrobial properties could be beneficial.

**Correct Answer:** D.