Figure 2317

Schematic diagram of the eye and lacrimal apparatus. This drawing shows the location of the lacrimal gland and components of the lacrimal apparatus, which drains the lacrimal fluid into the nasal cavity.

cells. Myoepithelial cells, located below the epithelial cells within the basal lamina, aid in the release of tears. Approximately 12 ducts drain from the lacrimal gland into the reflection of conjunctiva just beneath the upper eyelid, known as the fornix of the lacrimal sac.

Tears drain from the eye through lacrimal puncta, the small openings of the lacrimal canaliculi, located at the medial angle. The upper and lower canaliculi join to form the common canaliculus that opens into the lacrimal sac. The sac is continuous with the nasolacrimal duct that opens into the nasal cavity below the inferior turbinate. A

pseudostratified ciliated epithelium lines the lacrimal sac and the nasolacrimal duct.

Tears protect the corneal epithelium and contain antibacterial and ultraviolet-protective agents

Tears keep the conjunctiva and corneal epithelium moist and wash foreign material from the eye as they flow across the corneal surface toward the medial angle of the eye (Fig. 23.17). The thin film of tears covering the corneal surface is not homogenous, but represents a mixture of products secreted by the lacrimal glands, the accessory lacrimal glands, the goblet cells of the conjunctiva, and the tarsal glands of the eyelid. It contains proteins (tear albumins, lactoferrin), enzymes (lysozyme), lipids, metabolites, electrolytes, and drugs, the latter secreted during therapy.

The tear cationic protein lactoferrin increases the activity of various antimicrobial agents such as lysozyme.

The eye is moved within the orbit by the extraocular muscles

Six muscles of the eyeball (also called extraocular or extrinsic muscles) attach to each eye. These are the medial, lateral, superior, and inferior rectus muscles and the superior and inferior oblique muscles. The superior oblique muscle is innervated by the trochlear nerve (cranial newe IV). The lateral rectus muscle is innervated by the abducens newe (cranial newe VI). All of the remaining extraocular muscles are innervated by the oculomotor nerve (cranial newe III). The combined, precisely controlled action of these muscles allows vertical, lateral, and rotational movement of the eye. Normally, the actions of the muscles of both eyes are coordinated so that the eyes move in parallel (conjugate gaze).

The human eye is a complex sensory organ that provides sight. The wall of the eye consists of three concentric layers or coats: the retina, the inner layer; the uvea, the middle or vascular layer; and the comeosclera, the outer fibrous layer. The eye is often compared to a simple camera with a lens to capture and focus light, a diaphragm to regulate the amount of light, and film to record the image. In the eye, the cornea and lens concentrate and focus light on the retina. The iris, located between the cornea and lens, regulates the size of the pupil through which light enters the eye. Photoreceptor cells (rods ancl cones) in the retina detect the intensity (rods) and color (cones) of the light that reaches them and encode the various parameters for transmission to the brain via the optic nerve (cranial nerve II).

The eye measures -25 mm in diameter. It is suspended in the bony orbit by six extrinsic striated muscles that control its movement. The extraocular muscles are coordinated so that both eyes move synchronously, with each moving symmetrically around its own central axis. A thick layer of adipose tissue partially surrounds and cushions the eye as it moves within the orbit.

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