What is the third eyelid of a cat? - briefly
The third eyelid of a cat, also known as the nictitating membrane or haw, is a thin, translucent or opaque membrane located in the inner corner of the eye. It provides additional protection and helps to keep the eye moist and free from debris.
What is the third eyelid of a cat? - in detail
The third eyelid, also known as the nictitating membrane or haw, is a specialized anatomical feature found in cats and many other animals. This semi-transparent or translucent membrane is located in the inner corner of the eye and can move horizontally across the eye. Its primary function is to provide an additional layer of protection for the eye. When a cat blinks, the third eyelid helps to moisten and clean the eye, removing debris and preventing infections.
The third eyelid is composed of a thin layer of tissue that contains conjunctiva, which is the same tissue that lines the inner surface of the eyelids and covers the white of the eye. This membrane is rich in lymph nodules, which are part of the immune system and help to fight off infections. Additionally, the third eyelid contains a gland that produces a significant portion of the eye's tear film, contributing to the overall health and lubrication of the eye.
In cats, the third eyelid is usually not visible when the animal is awake and alert. However, it can become more prominent under certain conditions. For example, during sleep or when a cat is relaxed, the third eyelid may partially cover the eye. It can also be seen more clearly when a cat is sick or stressed, as the membrane may protrude slightly due to reduced muscle tone or inflammation.
The third eyelid can also be a sign of an underlying health issue if it appears pink, swollen, or prolapsed. This condition is known as "cherry eye" and occurs when the gland associated with the third eyelid prolapses and becomes visible. Cherry eye is more common in certain breeds and typically requires veterinary intervention to correct.
In summary, the third eyelid in cats serves multiple purposes, including protection, lubrication, and immune defense. Understanding its function and appearance can help cat owners recognize potential health issues and ensure the well-being of their pets.