Tears protect the corneal epithelium and contain antibacterial and UV-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 (see Fig. 24.17). The thin film of tears covering the corneal surface is not homogeneous but 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. The tear film 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.
FIGURE 24.17 • 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.
The eye is moved within the orbit by coordinated contraction of 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 nerve IV). The lateral rectus muscle is innervated by the abducens nerve (cranial nerve VI). All of the remaining extraocular muscles are innervated by the oculomotor nerve (cranial nerve 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 (called 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 corneosclera, 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 and 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.
Modifed drawing of human eye, meridional perspective by E. Sobotta.
The innermost layer is the retina (R), which consists of several layers of cells. Among these are receptor cells (rods and cones), neurons (e.g., bipolar and ganglion cells), supporting cells, and a pigment epithelium (see Plate 105). The receptor components of the retina are situated in the posterior three fifths of the eyeball. At the anterior boundary of the receptor layer, the ora serrata (OS), the retina becomes reduced in thickness, and nonreceptor components of the retina continue forward to cover the posterior or inner surface of the ciliary body (CB) and the iris (I). This anterior nonreceptor extension of the inner layer is highly pigmented, and the pigment (melanin) is evident as the black inner border of these structures.