11-29. THE ADNEXA
The adnexa are the various structures associated with the eyeball.
b. Eyelids. Attached to the margins of the orbit, in front of the
eyeball, are the upper and lower eyelids (palpebra (Latin), blepharon (Greek)). These have
muscles for opening and closing the eyelids. The eyelashes (cilia) are special hairs of
the eyelids which help protect the eyeball. The margins of the eyelids have special oil to
prevent the loss of fluids from the area. The inner lining of the eyelids is continuous
with the conjunctiva, a membrane over the anterior surface of the eyeball.
c. Lacrimal Apparatus. The conjunctiva must be kept moist and clean
at all times. To do this, a lacrimal apparatus is associated with the eyelids. In the
upper outer corner of the orbit is a lacrimal gland, which secretes a lacrimal fluid
(tears) into the junction between the upper eyelid and the conjunctiva. By the motion of
the eyeball and the eyelids (blinking), this fluid is moved across the surface of the
conjunctiva to the medial inferior aspect. Here, the lacrimal fluid is collected and
delivered into the nasal chamber by the nasolacrimal duct.
d. Eyebrow. The eyebrow (supercilium) is a special group of hairs
above the orbit. The eyebrow serves to keep rain and sweat away from the eyeball.
e. Optic Nerve (Cranial Nerve II). Neurons carry information from
the photoreceptors of the nervous retina. They leave the eyeball at the blind spot. At the
optic nerve, or second cranial nerve, the neurons pass to the rear of the orbit. There,
the optic nerve exits through the optic canal into the cranial cavity. Beneath the brain,
the optic nerves from both sides join to form the optic chiasma, in which half of the
neurons from each optic nerve cross to the opposite side. From the optic chiasma, the
right and left optic tracts proceed to the brain proper.
