| Accessory visual structures | |
| Latin: | structurae oculi accessoriae or adnexa oculi |
The accessory visual structures (or adnexa of eye, ocular adnexa, etc.) are the protecting and supporting structures (adnexa) of the eye, including the eyebrow, eyelids, and lacrimal apparatus. The eyebrows, eyelids, eyelashes, lacrimal gland and drainage apparatus all play a crucial role with regards to globe protection, lubrication, and minimizing the risk of ocular infection.[1] The adnexal structures also help to keep the cornea moist and clean.
One source defines "ocular adnexa" as the orbit, conjunctiva, and eyelids.[2] The orbit and extraocular muscles allow for the smooth movement of the eyeball.
See main article: Eyebrow. The eyebrow is an area of thick, short hairs above the eye. The main function is to prevent sweat, water, and other debris falling into the eye, but they are also important to human communication and facial expressions.
See main article: Eyelid. An eyelid is a thin fold of skin that covers and protects the eye. The levator palpebrae superioris muscle helps in the movement of eyelid. The human eyelid features a row of eyelashes along the eyelid margin, which helps in protection of the eye from dust and foreign debris. The main function of eyelid is to keep the cornea moist and clean.
See main article: Eyelash. The eyelashes are the numerous hairs that grow along the eyelid margin and are highly sensitive to touch. They function to heighten protection of the eye by catching dust and debris via blinking, and to regulate evaporation of the tear film of the cornea. Eyelashes are also important in human society and beauty standards, as prominent lashes draw attention to the eyes and thus are a universally desirable facial feature.
See main article: Conjunctiva. The conjunctiva is a tissue that lines the inside of the eyelids and covers the sclera. It is composed of unkeratinized, stratified squamous epithelium with goblet cells, and stratified columnar epithelium. The conjunctiva is basically transparent, and the white colour we see is actually sclera.
See main article: Lacrimal apparatus. The lacrimal apparatus is the physiological system containing the orbital structures for tear production and drainage.[3]
It consists of:
See main article: Orbit (anatomy). The orbit is the cavity or socket of the skull in which the eye and its appendages are situated. In the adult human, the volume of the orbit is 30ml, of which the eye occupies 6.5ml.[4] The orbit helps in smooth rotation of the eyeball.