The Adnexa

The adnexa are the various structures associated with the eyeball.

a. Extrinsic Ocular Muscles. Among the adnexa are the extrinsic ocular muscles, which move the eyeball within the orbit. Each eyeball has associated with it six muscles made up of striated muscle fibers.

(1) Four recti. Four of these muscles are straight from the rear of the orbit to the eyeball. They are therefore known as the recti muscles (RECTUS = straight). Each name indicates the position of the muscles in relationship to the eyeball as follows:

lateral rectus M. (on the outer side) superior rectus M. (above) medial rectus M. (on the inner side) inferior rectus M. (below)

(2) Two obliques. Two muscles approach the eyeball from the medial side and are known as the superior oblique and inferior oblique muscles.

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.

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