**Core Concept:** Meibomian glands are important for maintaining the integrity of the ocular surface. They are glands found in the palpebral and bulbar conjunctiva, producing an oily substance called meibum. Meibum is essential for the formation of a stable tear film on the ocular surface, preventing tear film instability and ocular surface damage.

**Why the Correct Answer is Right:** Meibomian glands secrete meibum, which is a complex mixture of lipids including cholesterol, free fatty acids, and their esters. These lipids play a crucial role in the tear film's structure and function. The tear film has three layers: an aqueous layer, a mucin layer, and the lipid layer produced by the meibomian glands. The lipid layer prevents the aqueous and mucin layers from mixing, maintaining the stability of the tear film and protecting the ocular surface from damage.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** This option is incorrect because meibomian glands do not produce sweat. Sweat is produced by the eccrine glands, which are located in the dermis, primarily responsible for regulating body temperature and electrolyte balance.

B. **Option B:** This option is incorrect because meibomian glands secrete meibum, not mucin. Mucin is produced by the goblet cells in the conjunctiva, which help to bind water to the tear film and prevent its rapid evaporation.

C. **Option C:** This option is incorrect as meibomian glands secrete meibum, not mucopolysaccharides. Mucopolysaccharides are secreted by goblet cells in the conjunctiva, which contribute to the formation of the mucin layer in the tear film.

D. **Option D:** This option is incorrect because meibomian glands secrete meibum, not saliva. Saliva is produced by the salivary glands, which are located in the salivary glands of the oral cavity, playing a role in digestion, taste, and oral hydration.

**Clinical Pearl:** Understanding the role of different ocular surface structures, like meibomian glands and their products, is essential for diagnosing and managing ocular surface disorders, such as dry eye disease, which can result from dysfunction of meibomian glands or other ocular surface structures.