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.