Anatomy and Physiology: Special Senses – The EYES
- Conjunctiva. The conjunctiva is a mucous membrane that begins at the edge of the cornea and lines the inside surface of the eyelids and sclera, which serves to lubricate the eye. It is the thin, transparent tissue that covers the outer surface of the eye. This structure is nourished by tiny blood vessels that are nearly invisible to the naked eye. The conjunctiva is composed of 3 sections:
- Palpebral Conjuctiva – covers the posterior surface of the eyelids
- Bulbar Conjuctiva – coats the anterior portion of the eyeball
- Fornix – the transition portion, forming the junction between the posterior eyelid and the eyeball
NOTE: Although the palpebral conjunctiva is moderately thick, the bulbar conjunctiva is very thin. The latter also is very movable, easily sliding back and forth over the front of the eyeball it covers. Since it is clear, blood vessels are easily visible underneath it.
Within the bulbar conjunctiva are “goblet cells,” which secrete “mucin.” This is an important component of the pre-corneal tear layer that protects and nourishes the cornea.
- Lacrimal Apparatus. The lacrimal apparatus consists of the lacrimal gland and a number of ducts that drain the lacrimal secretions into the nasal cavity. Located above the lateral end of each eye are the lacrimal glabds that continually release a dilute salt solution, known as tears, onto the anterior surface of the eyeball through several small ducts. The flow of tears flush across the following structures orderly:
- Lacrimal canals medially
- Lacrimal sac
- Nasolacrimal duct that empties into the nasal cavity.
Lacrimal secretion contains antibodies and an enzyme that destroys bacteria known as lysozyme. Hence, tears cleanse and protect the eye surface as it moistens and lubricates it. in cases when lacrimal secretion substantially increases, tears spill over the eyelids and fill the nasal cavities. This causes congestion and the “sniffles.” When eyes are irritated by foreign objects or chemicals and when a person is emotionally upset lacrimal secretion is stimulated and increased.
Internal Eye structures
The eye, commonly called the eyeball, is a hollow sphere and is composed of:
- 3 tunics or coats
- Humors – the interior filled with fluids that help maintain the shape of the eye
- Lens – the main focusing apparatus of the eye. This structure is supported within the eye cavity dividing it into two chambers.