The eyes have various functions that are controlled by muscles. The extraocular muscles, which are attached externally, move the eyes towards the appropriate visual target. These muscles are innervated by the oculomotor nerve (cranial nerve [CN] III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). Additionally, there are intraocular muscles within the eye itself. The muscles in the ciliary body control the shape of the lens, which affects the focus of images on the retina. The iris contains the pupillary dilator and sphincter muscles, which regulate the amount of light entering the eye similar to a camera diaphragm. The dilator muscle is activated by the sympathetic nervous system, while the sphincter and ciliary muscles are controlled by the parasympathetic nervous system through the oculomotor nerve. Light enters the eye through the cornea and passes through transparent fluids and structures known as the dioptric media, which include the cornea, aqueous humor, lens, and vitreous humor. The aqueous humor is found in the anterior and posterior chambers, while the vitreous humor is located behind the lens. These fluids help maintain the shape of the eye. The geometrical optic axis of the human eye passes through the nodal point of the lens and reaches the retina between the fovea and the optic disc. However, the eyes are oriented towards a fixation point on the visual target by the oculomotor system. Light from the fixation point is focused on the fovea, while light from the rest of the visual target falls on the surrounding area of the retina. Normally, the cornea and lens refract light to focus it sharply on the retina. The cornea is the main refractive element with a refractive power of 43 diopters (D), while the lens can change shape and adjust its refractive power between 13 and 26 D. The lens is held in place by suspensory ligaments (zonule fibers) attached to the ciliary body. When the ciliary muscles relax, the tension from the suspensory ligaments flattens the lens. On the other hand, when the ciliary muscles contract, the tension is reduced, allowing the lens to assume a more spherical shape. The ciliary muscles are activated by the parasympathetic nervous system via the oculomotor nerve. This mechanism allows the eye to focus on both near and distant objects. For instance, when a distant visual target is observed, the image is in focus on the retina. However, if the eye is directed at a nearby visual target, the initial focus is behind the retina, causing the image to appear blurred until accommodation occurs. A diopter is a unit of measurement for optical power, equal to the reciprocal of the focal length measured in meters. Therefore, it is a unit of reciprocal length, and a 2-D lens would bring parallel rays of light into focus at a distance of 0.5 m.
A number of functions of the eyes are under muscular control. Externally attached extraocular muscles aim the eyes toward an appropriate visual target (see ). These muscles are innervated by the oculomotor nerve •Fig. 8.1 Illustrationofaviewofahorizontalsectionoftherighteye.(RedrawnfromWallGL.The Vertebrate Eye and Its Adaptive Radiation. BloomfieldHills,MI:CranbrookInstituteofScience;1942.)IrisPosterior chamber Anterior chamber Limbus Sclera Choroid Retina Optic disc Fovea Vitreous humor Optic nerve Temporal retina Nasal retina Ciliary body Zonule fibers Nodal point Lens Aqueous humor Cornea (cranial nerve [CN] III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). Several muscles are also found within the eye (intraocular muscles). The muscles in the ciliary body control lens shape and thereby the focus of images on the retina. The pupillary dilator and sphincter muscles in the iris control the amount of light entering the eye, in a way similar to that of the diaphragm of a camera. The dilator is activated by the sympathetic nervous system, whereas the sphincter and ciliary muscles are controlled by the parasympathetic nervous system (through the oculomotor nerve; see
Light enters the eye through the cornea and passes through a series of transparent fluids and structures that are collectively called the dioptric media. These fluids and structures consist of the cornea, aqueous humor, lens, and vitreous humor (see
Fig. 8.1 ). The aqueous humor (located in the anterior and posterior chambers) and the vitreous humor (located in the space behind the lens) help maintain the shape of the eye.
Although the geometrical optic axis of the human eye passes through the nodal point of the lens and reaches the retina at a point between the fovea and the optic disc (see
Fig. 8.1 ), the eyes are oriented by the oculomotor system to a point, called the fixation point, on the visual target. Light from the fixation point passes through the nodal point of the lens and is focused on the fovea. Light from the remainder of the visual target falls on the retina surrounding the fovea.
Normally, light from a visual target is focused sharply on the retina by the cornea and lens, which bend or refract the light. The cornea is the major refractive element of the eye, with a refractive power of 43 diopters (D). However, unlike the cornea, the lens can change shape and vary its refractive power between 13 and 26 D. Thus the lens is responsible for adjusting the optical focus of the eye. Suspensory ligaments (or zonule fibers) attach to the wall of the eye at the ciliary body (see
Fig. 8.1 ) and hold the lens in place. When the muscles in the ciliary body are relaxed, the tension exerted by the suspensory ligaments flattens the lens. When the ciliary muscles contract, the tension on the suspensory ligaments is reduced; this process allows the somewhat elastic lens to assume a more spherical shape. The ciliary muscles are activated by the parasympathetic nervous system via the oculomotor nerve.
In this way, the lens allows the eye to focus on, or accommodate to, either near or distant objects. For instance, when light from a distant visual target enters a normal eye (one with a relaxed ciliary muscle), the target image is in focus on the retina. However, if the eye is directed at a nearby visual target, the light is initially focused behind the retina (i.e., the image at the retina is blurred) until accommodation aA diopter is a unit of measurement of optical power that is equal to the reciprocal of the focal length measured in meters. Thus it is a unit of reciprocal length, and a 2-D lens would bring parallel rays of light into focus at a distance of 0.5 m.