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The structure of the human eye
The eyeball is generally spherical, with a diameter of about 2.5 cm, loosely embedded in the fat tissue in the cone-shaped cavity of the skull. Each eyeball has six external muscles, which work in pairs to determine the position of the eye. A pair of lateral muscles controls lateral movement; the upper and lower muscles control upward and downward movement; the two oblique muscles are mainly used to produce rotational movement around the visual axis. Adjusting the movement of the external muscles of the two eyeballs can keep the visual axis converged on the gaze point at any distance.
The outer layer of the eyeball is a protective fiber, about one millimeter thick, called the "sclera". In the front part of the eyeball, which accounts for about the total area, there is a layer of uniform thickness, neatly arranged, and thus transparent fiber film. This layer of transparent film is called the "cornea", and its radius of curvature is slightly smaller than the rest of the outer layer.
The second layer of membrane inside the sclera, called the "choroid", is a thin film composed of arteries and capillaries. Generally speaking, it is used to supply nutrition to the eyeball, specifically to the nerves. Near the cornea, the choroid is replaced by the ciliary body, where the muscles of the eye grow outside.
Inside the choroid is a third layer, called the retina. The retina contains nerve receptors that respond to light. The part of the retina in the visual axis of the eye, called the fovea, is particularly important because the image of the center of gaze is formed here.
Visual Pathway Light enters the eye through the cornea and passes through a cavity filled with a clear fluid, the aqueous humor, and then through a colored diaphragm, the iris, with a hole in the iris that is the pupil. The diameter of the pupil is about 2 to 8 millimeters, depending on the brightness of the field of view. The brighter the field of view, the smaller the pupil. The control of the iris is accomplished by the circular sphincter and radial dilator muscles, which grow in the ciliary body and are composed of one of two groups of muscles in the eye. The control of the iris is not arbitrary.
Then, the light passes through a lens, which is composed of multiple layers of fibers wrapped in an elastic outer capsule like an onion. The external capsule normally flattens the lens by holding the lens in place under tension from a thin membrane composed of radially suspended ciliary ligaments. The outer ends of the radial ligaments are attached to a portion of the ciliary body, a part of the circular ciliary muscle. When looking at a close object, the muscle contracts, reducing the radial tension of the ciliary ligaments and causing the lens to thicken. This process is called "accommodation". The space between the lens and the retina is filled with a transparent gel called the vitreous body.
Retina The retina is the end of the light path in the eye and the beginning of the optic nerve, which in turn ends in the visual cortex, the part of the brain that regulates vision. The retina occupies about two-thirds of the inner surface of the eyeball and consists of: (1) a layer of two types of receptor cells, called cones and rods according to their shape, which is close to the choroid; (2) a layer of bipolar cells; and (3) a layer of ganglion cells, whose optical nerve fibers lead to the brain.
These cells and fibers are translucent and must be transparent because they are located in front of the light-sensitive receptor cells. There are about 100 million photosensitive cells (about 6-7 million of which are cones) and 1 million nerve fibers in the eyeball. Most of the bipolar cells are connected to one or more receptor cells and nerve fibers, but this connection is complex, including a network of receptor cells connected by so-called horizontal bipolar cells.
The nerve fibers leave the eye from the blind spot, where there are no receptor cells. Apart from the blind spot, the distribution of receptor cells on the retina is not uniform, but their distribution is roughly symmetrical to the visual axis of the eyeball. The cells and nerve fibers on the visual axis lean sideways, forming a pit on the inner surface of the retina, the fovea, which shows the characteristics of increased density of rod cells and decreased density of cone cells throughout the area 1/2 of the visual axis. This 1/2 area determines the edge of the depressed part of the fovea. About 20° away from the visual axis, the density of rod cells increases to a maximum of about 160,000 per square millimeter, while the density of cone cells decreases to a minimum of about 5,000 per square millimeter and remains to the edge of the retina. Each bipolar cell on the edge connects a large number of rod-shaped or cone-shaped sensory cells to a single optic nerve fiber.
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